I would prefer immediate death to a thousand subjective years of agony followed by death, if that were all that was at stake. And I'm pretty sure that after, say, a subjective year of agony, I'd happily kill off an otherwise immortal copy of me to spare myself the remaining 999 years.
But I estimate that to be true because of the emotional stress associated with extreme pain, not because of any sober cost/benefit analysis. Absent that stress, I quickly conclude that any given moment of agony for Gest(a) is worth an arbitrary but much larger number of pain-free moments of continued existence for Gest(b); and given that I'd consider both branches "me" relative to my current self, I don't find myself with any particular reason to privilege one over the other.
People undergoing torture will betray their strongest beliefs, even sell out their friends into precisely the same fate, to get the pain to stop. Well-known. But I don't think this reflects their true volition for any reasonable value of "true": only a product of extreme circumstances which they'd certainly regret later, given the opportunity.
If you could gain immortality by subjecting someone else, a stranger from a different country, someone you've never met, to a thousand years of torture--would you do it?
This introduces issues of coercion which don't exist in the original problem, so I don't believe it's a fair analogy -- but my answer is "only with their consent". I would consider consenting to the reverse under some circumstances, and I'd be more likely to consent if the world was about to end as per the OP. I'd immediately regret it, of course, but for the reasons given above I don't believe that should influence my decision.
I'm just not sure about the way you're discounting the preferences of Nornagest(torture1000). In my imagination he's there, screaming for the pain to stop, screaming that he takes it back, he takes it back, he takes it back. His preferences are so diametrically opposed to "yours" (the Nornagest making this decision now) that I almost question your right to make this decision for him.
I have been trying to absorb the Lesswrong near-consensus on cryonics/quantum mechanics/uploading, and I confess to being unpersuaded by it. I'm not hostile to cryonics; just indifferent, and having a bit of trouble articulating why the insights on identity that I have been picking up from the quantum mechanics sequence aren't compelling to me. I offer the following thought experiment in hopes that others may be able to present the argument more effectively if they understand the objection here.
Suppose that Omega appears before you and says, “All life on Earth is going to be destroyed tomorrow by [insert cataclysmic event of your choice here]. I offer you the chance to push this button, which will upload your consciousness to a safe place out of reach of the cataclysmic event, preserving all of your memories, etc. up to the moment you pushed the button and optimizing you such that you will be effectively immortal. However, the uploading process is painful, and because it interferes with your normal perception of time, your original mind/body will subjectively experience the time after you pushed the button but before the process is complete as a thousand years of the most intense agony. Additionally, I can tell you that a sufficient number of other people will choose to push the button that your uploaded existence will not be lonely.”
Do you push the button?
My understanding of the Lesswrong consensus on this issue is that my uploaded consciousness is me, not just a copy of me. I'm hoping the above hypothetical illustrates why I'm having trouble accepting that.