Specifically, what percentage of bowl teams had a better record the following year, compared to the bowl teams that didn't go to a bowl game.
I still suspect it'll break even. I don't think extra practices are likely to offer a significant impact on the next year's performance, one year or another. But I agree, unless someone has a career ending injury, it can't hurt.
In my opinion if the players look forward to going in a year like this one, they should get to go. If they'd rather go home to their families, then that's probably the best use of their time.