What do I gain by going to 'performance' front end parts?

Unless you are upgrading your wheel and tire package along with performance shocks just go with stock stuff because you won't see much difference.

The upper "A" arms will allow for more alignment freedom but you can also get some of that with offset upper bushings in your stock A-arms.

I know this is overly simplified but if you are running some craptastic 14" or 15" white letter tire and plan on continuing to run a tire like that you just won't be able to take advantage of these upgraded suspension components (or the gain will be rather small).