If you are starting your PrC as early as you can (usually 6th or so), then you should be able to expect having the final end cap abilities of the class by the end game.
There's a big caveat though. If your character build really doesn't kick in until 10th+ level, then you'll end up playing through most of the game "waiting for your character concept to kick in", which is not really that fun.
Kingmaker is especially good for giving a lot of hooks for further adventuring after the APs are done though, so it can be a great kickstarter to high level gameplay if the DM is up to making his own stuff.
I'd even go so far as to say that it's a greater primer for an Epic game, you end up with a Kingdom, after all.