The Cupra with 312 has too much initial bite and is hard to get a smooth stop when coasting at 1-2mph up to lights but I have know even worse (Volvo traditionally and Cophen today). IMHO as a brake engineer it is "over servoed" already but then so are most cars today. Very hard to get a progressive set in a car today as they are all designed to be driven in high heels in traffic (!!) and not on a fast A road with good brake balance and control.
Anyway the brake assist just allows air pressure in the back of the servo to pull the brakes on .. MB designed it 20 years ago when I still did brakes! It will not give you grabbier brakes;
1. Better pads can
help - cheap ones do not grab well.
2. 312mm calipers will
help
3. metal braided hoses will make it firmer at medium braking
4. Bigger servo or smaller master cylinder would do something after lots of engineering..
Never had FR so not sure how poor they are.. suspect they are VAG typical i.e. not the best but adequate. The Cupra 4 pots really are very effective but somehow my old VR6 Golf with 312 cast iron set up was a better setup for brakes.