Goal is to pull a QV for $10k or less in parts. Safely.
And you'll only be missing the bigger brakes to better match the power, torque vectoring diff, bigger trans, active aero, the many aesthetic differences, race mode (ESC/TC defeat), wider wheels/rubber, etc. And even if you manage to pull a QV, the QV with a $800 piggyback will pull you right back.
I know the "just buy a QV" rhetoric annoys some people, but the price gaps aren't that huge, unless you are buying a stripped 2.0. And it might explain part of the reason for the lack of this type of aftermarket.
That's your kind of thinking. As long as I can trap 120+mph I'll be happy and content. You're forgetting the compounding cost of ownership; insurance, maintenance, mpg, weight, QV parts/tuning tax, and at the end of the day it's just a QV nothing out of the ordinary, just expected to be fast. If a Q2/Q4 pulls a QV regardless if stock that is something special.
Power- bigger turbo
Torque vectoring- love my old school Ti Sport mechanical LSD. So mechanical>electric control LSD.
Brakes- being that there is less weight on the front axle the 4 pots are just fine with rotors, pads and fluid change. Smaller allows to run 18" wheels all around and 17" in back if wanted for drag wheels.
Wheels- individualized with lighter Forged ones of your choice.
Tires- again your choice.
Aero- rather have static than active. Lighter and less costly to repair. Can individualize for purpose if racing.. or style.
Transmission- zf8hp50 is well enough for 700whp/600wtq heavier 6cyl turbo BMWs
ESC/TC- can be coded to be off or less invasive see EuroCompulsion.
IMHO I think the 2.0T Ti Sport is the best model and greater potential for modification/tuning. The chassis overall is a highway cruiser and canyon carver. The balance of the 2.0T in this chassis you can feel in the front.
I see an ongoing theme with purists/elitists/whoever saying just get the QV and do nothing to it. That is boring and the very reason I skipped on the last 2 M3 cars, I was faster and different with the last 2 335i models I owned.
We have differing opinions and tastes. Don't stifle others if it doesn't jive with your own.