Awesome! This question bears directly on the intersection of thermodynamics, computational complexity theory, economics, and identity instantiation, and hence I cannot resist answering.

The question boils down to the relave scalability of different effects with respect to each other:

1) How much computation can you produce per unit energy?
2) How much utility (or how many happy-people-equivalents) can you produce per unit of computation?

If reversible computing is possible, then the answer to 1) is that you can produce as much as you want, producing arbitrarily small energy losses (due to the inability to achieve perfect irreversibility e.g. due to friction in implementation.)

This would render pessimism about 2 irrelevant, it seems.

Edit: And furthermore, if we accept the notion that a virtual being's subjective experience of time depends soley on the program that instantiates them (e.g. a species of substrate independence that is widely accepted here), then such beings would subjectively live forever, even if reversible computing is constrained by having to be arbitrarily slow. (Although people would have to accept uploading themselves, of course.)