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

All depends on what you are looking for. Just a basic OE replacement over a worn out set will make a huge difference. If you are running it hard and want optimal performance, like 70 Duster said, go with tubular arms with hi performance bushings etc. I have the RMS complete front end and it's awesome stuff if you have the budget or just do a coil over swap with stock K. Lots of options just weigh your budget just like anything else. Good luck