Welcome to our forums,
This is one of those questions that gets thrown around alot. Ill try to help you sort it out.
First thing - Subaru of American CANNOT void your warranty for any reason. There is no void warranty checkbox in their system and nothing you could do to the car would void the warranty. What the CAN do is deny a warranty claim if they determine that your use was outside the "Normal Use" of the vehicle. Installing a 5th wheel hitch and towing a Semi trailer on the 15 to vegas might be a reason for them to deny a transmission or brake warranty claim. To give you a more reasonable example installing an aftermarket exhaust system and changing the factory tune is outside the "Normal Use" as defined by Subaru and they may choose to deny warranty claims against the engine if they determine that your mods caused the problem or exacerbated it. 1 final example I have a Stage 2 ( Exhaust, Tune, intake and a laundry list of suspension and braking mods ) and SOA recently replaced my cloth seat cover because it was coming apart at the seams. My mods had nothing to do with my seats fabric coming apart so they handled it without any questions asked.
As far as mods you can do that will increase performance and not risk Subaru of America choosing to deny warranty claims I would start with some good performance Rubber, a slightly wider contact patch, stiffer sidewalls and lighter wheels if you have it in your budget.
Hope that helps
Slow is Smooth > Smooth is Fast