If you're going to shut up and calculate, you need to calculate not only the benefit and harm from the experiment, you need to calculate the benefit and harm from weakening the ethics code. You can't weaken ethics just for one experiment; if you weaken it for experiments where the benefit outweighs the harm, you'll also weaken it for experiments where the harm outweighs the benefit but the people performing the experiment value harm to some people more than to others, and for experiments where the experimenters just don't know how to calculate, and for experimenters where an unspoken goal of the experiment is to actually cause harm, and....
Smoking is a situation where any remotely accurate model of the situation says that the value of information is extremely high inasmuch as smoking was one of the most popular activities in the world at the time, was expected to continue be so, the correlates (if causal) translated to enormous loss of life, and since it took decades to do so, the correlative evidence was obviously too weak & unpersuasive to motivate people to quit or overcome societal presumptions/precommitments against heavy regulation & bans.
Against this obvious point, you set - as even more desirable - preserving a bunch of made-up incoherent* rules or 'ethics' put into place post-WWII as a response to the Nazi concentration camp tortures and things like the Tuskegee syphilis experiment, none of which activities would have passed previously-accepted norms of conduct or even scientific reporting (eg. were any of Mengeles's 'experiments' done adequately and could reach any level of scientific validity? the only halfway useful ones I know of were the altitude experiments), and were done, surprise surprise, under conditions of secrecy and the auspices of totalitarian dictatorships and the closest thing to that in the West, militaries. But you know what, having IRBs and 'ethics' didn't stop stuff like MKULTRA post-WWII, did it? That's because secrecy is the father of abuse, not any 'lack of research ethics'. You're solving the wrong problem. The problem is not that scientists in their ordinary course of conduct investigating things such as smoking are monsters. The problem is that governments and militaries and corporations and elites are monsters who, if you let them, will happily do monstrous things and attract and enable and cover up monsters like Mengele and try to ignore things like 'downwinders'.
* I find 'informed consent' particularly hilarious, having participated in a few experiments at college. At no point was I ever 'informed' in any meaningful way about risks with, for example, a specific probability like 'based on a meta-analysis of previous experiments, we think there's a <5% chance you'll have a rash or worse'. No wonder the bioethics journals continue debating how many angels can dance on the head of a pin - I mean, how informed consent applies to Third World clinical trials, old people, reuse of data for additional analyses, etc etc.
So yes, I am perfectly happy to bite your bullet and advocate for undermining your 'ethics' and things like IRBs. They are worthless. They do nothing but impede science, and trade off endless sins of omission in exchange for (possibly) preventing some sins of commission. This is nothing like courts, and precommitments do nothing here but harm.
The willingness to settle for trash correlations massively harms science and society (one word, which should silence all doubters of this: "diet"), and to the extent that a smoking experiment weakens this norm and helps people consider the long run & inherent uncertainty of even the best correlative evidence, I regard it as an unalloyed good above and beyond the issue of tobacco.
I hope that one day, the question people ask will never be, 'could it possibly be moral to run a randomized experiment on X?', but rather, 'could it possibly be moral to not run a randomized experiment on X?' (or better yet, they don't ask the question at all since it is taken for granted that, not being superstitious barbarians, of course a randomized experiment has been run).
one word, which should silence all doubters of this: "diet"
Is it not allowed to run randomized controlled trials assigning diets to people? I'm pretty sure I've read about such trials. Do ethics boards forbid assigning diets they (without good evidence) believe to be harmful?
A look at all natural foods through the lenses of Bayesianism, optimisation, and friendly utility functions.
How should we consider foods that claim to be "all natural"? Or, since that claim is a cheap signal, foods that have few ingredients, all of them easy to recognise and all "natural"? Or "GM free"?
From the logical point of view, the case is clear: valuing these foods is nothing more that the appeal to nature fallacy. Natural products include many pernicious things (such as tobacco, hemlock, belladonna, countless parasites, etc...). And the difference between natural and not-natural isn't obvious: synthetic vitamin C is identical to the "natural" molecule, and gene modifications are just advanced forms of selective breeding.
But we're not just logicians, we're Bayesians. So let's make a few prior assumptions:
Now let's see the food industry as optimising along a few axis:
The last point is the weakest one, though. Long term health impacts are fiendishly hard to measure (by anyone), and consumers' memories are short. The main things that the food industry wants to avoid are blaring headlines like "salmon saturated with mercury", "sugar linked to cancer", or "food industry coverup". Even there, journalists can create scandals out of very little (or ignore major ones if something more juicy comes along), so the pressure to actually prioritise long term consumer health is very weak. So most optimisation pressure is along the first two axis.
So the food industry will strongly push to decrease cost and increase satisfaction, while mostly taking a random walk on long term consumer health. Given the assumptions above, this means that we'd expect the long term health impacts to worsen (because there are far more negative products than good one). This somewhat similar to the importance of programming everything of value into a friendly AI, lest the things not programmed get squeezed out.
Now, what does "all natural" mean in this setting? It means that the natural food industry is facing a massively constrained optimisation problem. They are extremely limited in what they can do (compared with the rest of the industry), and it mostly involves shuffling around with products we suspect to be benign or positive. Similarly with GM modifications: selective breeding is much slower and uncertain, so the optimisation pressure is less: they literally can't change things as much or as fast. One supporting argument for this is that all natural or GM-free products tend to be more expensive or less satisfying than others, demonstrating less optimisation pressure.
This is entirely not a result I expected to find. Pushing it to the extreme, it would seem that the most traditional and unchanged food (after removing stuff we know to be bad) is likely to be best, as long as people don't get too inventive with them.