Those items do not void the warranty so long as they are Genuine Subaru Accessories. Just know that accessories installed AFTER your purchase are NOT covered by the 3yr, 36K mile warranty, only the 90 warranty applies to them.
I think Duderotomy was afraid you were going to do engine modifications.
You can install any part on your car either yourself or by using local mechanic or dealer. Dealer will have to prove that what you did on your own outside of their shop caused any damage to have basis for voiding your warranty. Most of the times if you mod things nothing happens and your warranty is still in place but it really depends on how much of an A-hole your dealership wants to be.
if what others are suggesting was the case, you wouldn't be able to change your brake pads or wheels without fearing that the warrantly will be voided. You don't have to use genuine subaru accessories either.
When I had my WRX it was fairly moded (stage 2 computer tune with supporting exhaust mods, different suspension etc.) and dealer had no problem fixing the clutch judder or fuel smell problem since these were issues that had nothing to do with my mods. Obviously they would look differently at blown turbo but that was something that I was able to live with.
Dealer cannot just void a warranty, there are laws protecting you as a consumer.