If I had to build a BRZ STI it would be simple. More power, upgraded suspension, lighter weight, and more aero. I know this has been done by many lately, but on a much smaller level. Lately when you see an "R" model or performance version in general, they throw on some big wheels, summer tires, slightly better spring rates, bigger sways, LSD, and a handful of cosmetic changes. Now when you hear all this you think its the best thing since sliced bread. Sadly these modifications are often minor and not worth the extra cash on the sticker price. More "bling" than "zing"
Lets talk about power. I know you power hungry guys aren't going to be happy without a turbo. So if a turbo is the only option for you, skip over this part. The FRS/BRZ makes 200hp out the box, roughly 160whp. If that number could be bumped to 200whp, that would be nearly perfect to power the light weight FRS. Upgrade to a lighter flywheel to increase throttle response and a higher final drive to get the car off the line faster. The transmission is great as it is. A short shifter would only be bonus points and the car is already equipped with a LSD.
On to the suspension. Given that I have not had any extensive seat time in the BRZ/FRS I cant give too much of an opinion on how I would upgrade the suspension, but sways would surely need to be changed. Upgraded springs and dampers with a slightly lower stance would be ideal, a bit too much dive in stock form for me. The wheels and tires would be the largest improvement to the suspension. Wider wheels and sticky summer tires would make a huge improvement in keeping the light weight car planted. The brakes a quite sufficient on the stock BRZ/FRS, some simple ducting to keep them cool would be adequate.
Now the weight. There isn't much to a BRZ/FRS to begin with so taking away weight won't be easy, but there are a few things that could lose some weight. Lighter wheels, ditching the useless backseat, remove cruise control, roll up windows, carbon roof, and the lack of spare tire and tools would all be beneficial. Optional radio and A/C packages should also be available.
As far as aero a more aggressive front and rear lip should do the trick. Nothing crazy like the typical STI wing, just simple clean and functional. Remember this is a $30k car, so you cant expect a company to spend millions on superior aero equipment.
This would be my ideal BRZ STI and in all reality it isnt much to ask for. The aftermarket is there to support near 200whp NA, suspension work they probably already have on the SPT self, and removing weight......well 2004 STI BBS bolt right up. It wouldn't take much to really make this car a reality. The 04 STI didn't even come with a radio. Miatas up until recently have always been optioned with or without power steering or A/C. The BRZ/FRS brings back a different car to the market. Hopefully something like this is put into production, or atleast on the drawing board. I've been waiting for a car to return with heart ,soul instead of trying to force tons of power into something to satisfy the public. So thats my ideal BRZ STI, whats yours?