Does Subaru's Sti or another company make performance upgrades that can be bought through dealer and installed by them so it will not void the warranty?

nope...
sti and spt products still come with a disclaimer of warranties...
and can indeed void the warranty on those specific parts...
(suspension/exhaust, etc)

http://www.subaru.com/microsites/spt/parts.sptmain

good luck
Jamie
www.subiegalracing.com

Thanks for the info.