I'm starting to doubt I know what 'sentience' means, if it means what you're using it to mean. Your argument doesn't really work given my definition of sentience, since I don't equate sentience and subjective experience.
I don't see why sentience is automatically given such important status in moral reasoning. I'd be more tempted to heavily weight things like consciousness/sapience. Are there good non-obvious arguments for privileging sentience somewhere? Feeling pain and pleasure is a big part of what we consider good and bad, but there are many valuable things that fall outside the scope of 'sentience' in my current worldview.
Something like what you're arguing seems correct, but I think the point is obvious-seeming to good reductionists and we should come up with better arguments for this obvious-seeming thing.
we should come up with better arguments
Here is my modest suggestion. Naturalistic ethics. A rational agent has moral significance if it (or its coalition) engages in Nash bargaining with you (or your coalition). That is, you should make nice to it, only if it rewards its benefactors, punishes its malefactors, and gives strangers the benefit of the doubt. The amount of good you should do for your coalition members ought to balance (at the margin) the good they do for you. Your coalition works best on the basis of complete honesty.
There are no other ...
It has been suggested that animals have less subjective experience than people. For example, it would be possible to have an animal that counts as half a human for the purposes of morality. This is an argument as to why that may be the case.
If you're moving away from Earth at 87% of the speed of light, time dilation would make it look like time on Earth is passing half as fast. From your point of reference, everyone will live twice as long. This obviously won't change the number of life years they live. You can't double the amount of good in the world just by moving at 87% the speed of light. It's possible that there's just a preferred point of reference, and everything is based on people's speed relative to that, but I doubt it.
No consider if their brains were slowed down a different way. Suppose you uploaded someone, and made the simulation run at half speed. Would they experience a life twice as long? This seems to be just slowing it down a different way. I doubt it would change the total amount experienced.
If that's true, it means that sentience isn't something you either have or don't have. There can be varying amounts of it. Also, someone whose brain has been slowed down would be less intelligent by most measures, so this is some evidence that subjective experience correlates with intelligence.
Edit: replaced "sentience" with the more accurate "subjective experience".