The Subaru extended warranty is a good deal, IMO, but don't pay retail. Also, get the SPT exhaust, but don't pay more than 1000.00 installed, and demand a 488.00 credit for your brand new stock exhaust. Have the dealer or mechanic replace the mounts with N mounts. Don't get the service package or Simonizing, etc.
Then, get some snows mounted on inexpensive (120.00 each) wheels, such as MSW, which are made by OZ, and some really good snow tires, like Verdensumtin, from Direct Tire. Ask for Gino in Watertown.
Then, take a ride to No. Kingston, RI and have M@ tune your car. Replace your stock sway bars with WL 24mm front and rear sway bars, replace your end links with WL or another good brand, install the AVO rear sway bar braces, install the RCE yellow/Bilstein spring/strut/shock package, install the WL roll center and adjustable rear camber kits, install a good A/O separator or a cheap $20.00 Caldwell catch can, have Scott in Peabody perform an alignment, install a GrimmSpeed master cylinder brace, perform the hack on your nav system, if you get one, if the system is anything like the 2011, install the WL subframe chassis bushing and rear differential kit, install (by recommendation from Numero Uno) the 35.00 ebay drive shaft bushing, and replace the feline Subaru horn with (4), at a minimum, Hellas. You'll rarely beep your horn again, except when absolutely necesary. The STi's HID low beams suck, so get a light bar and some driving lights from Rally Innovations. Save 165.00 and pass on the Rally Armor mud flaps.
You don't need chassis bracing, front or rear strut bars, 3 point solenoid boost control, cold air intake, etc., etc. Subarus are OK side-to-side. The drive-train needs to be tightened up front-to-back. You will notice a substantial increase in responsiveness.
Get a good radar detector and laser shifters. Wave @ every Subie that passes. Drive defensively. Cry when you get pulled over. GLHF. Be safe.
If you arent shopping @ Macnchester Subaru, bring the printed price sheet. If the dealer says "We cant possibly match those prices" get up and start leaving. When they ask where you are join, just say "Manchester". Dan, the manager @ Cityside, will price-match sometimes. YMMV, but bring a pizza and beer or munchkins with you.