Yeah they won't service it with pre-release software on. They do let you downgrade in store if you really need to, but better just to go in running iOS 7
Ex genius bar guy checking in, that's not completely true. For something like this that's a "known issue" (according to apple, not some forum) or obviously hardware, like dead pixels or something, they can service it no matter what software it's running.
The catch is that they may need to run the battery diagnostic on it before repairing it, and the diagnostic is likely not compatible with iOS 8. Also the repair process for this may require them to restore your phone anyway, like the sleep/wake button program. In that case it doesn't matter, because they're restoring it anyway. In the past, at least, beta software policy wasn't clearly communicated to the staff, so you may get someone who thinks they can't touch it as long as it has iOS 8, but if you stay chill and ask them to double check it should work out.
No matter what, always always always make a backup before setting foot in the store.
For everyone else with battery problems, especially if your phone doesn't qualify:
You may very well still have a bad battery, it's just not bad for the reason covered by the program. If your phone consistently dies at ~20-30%, that sounds like a battery that's going to fail the diagnostic. No promises, but make a backup and go in.
If you go in to the store, and your battery passes the diagnostic, that means your battery is physically ok. It may not be what it was when you first got it, but that's normal. Batteries age. If it passes diagnostics, that means it hasn't dropped below the cutoff where Apple will replace it, not matter what your experience with it is. In that case, replacing the battery WILL NOT solve the problem, because... the battery isn't the problem.
Honestly, the combination of short battery life/passes diagnostics almost always comes down to a mix of environmental issues and the usage of the phone. Poor cell reception, max brightness, constantly streaming pandora, etc. You know the hundred times a day you don't know what else to do, so you flip through facebook or play candy crush? That's why the battery is draining.
When customers came in with a battery problem that passed diags, about a quarter of the time auto-lock was turned off. It's things like that that get you.