Best brake setup that can fit under 15s?

[Borat]

Yes. Force/Area = Force/Area Basic Physical principle.

If you increase the area (going from the standard 20.64mm MC to a 25mm MC) you must increase the force proportionally. Therefore, you have to apply ~21% more pedal force to achieve the same amount of brake torque.

um, not sure what size stock MC is but assuming it is 20.64 then you need over 45% more brake force. Quite a bit more really.

First bit of advice is to just make sure everything is clean and well maintained. Then pads, rotors, lines etc

If that don't stop you well enough you can use 280mm x 22 VW rotors (cheap ~$35 for brembos), only require simple mod to fit. Make up a bracket to relocate your stock caliper or go for e28 calipers (still need a bracket) and MC. The rear e28 calipers have the same piston size as E30 so bias should be all good.

[IS]

um, not sure what size stock MC is but assuming it is 20.64 then you need over 45% more brake force. Quite a bit more really.

You're right, I forgot to take the area and instead did the ratio of the diameters!

The best way to improve braking, assuming your brakes are capable of generating enough torque to lock up the tires, is to get stickier tires.

Just recently, I went with hawk hp plus pads, ss lines, ate blue fluid, and brake ducts for the front rotors. The limiting factor of my setup is the tires - which are not crap. 225/45/16 hankook rs2s. There is a road by my house, called winding way, in which there are about 15 braking points there and back. I would fade the stock brakes before finishing it, if I really pushed it. Currently, I can do "2 laps" there and back and the brakes still hold up.

I haven't been able to get my e30 out on to the track yet, but a DE at Thunderhill will be indicative of the brakes performance.