Posted this in another thread, but very relevant.
If you are looking for a true upgrade that is cost effective, I'd suggesting calling up Chris at Red Shift Motorsports:
https://store.redshiftmotorsports.com/
He starts with BC Coilover bodies, guts them and rebuilds them custom for the application using very high-end parts. They are used by some of the fastest cars at TT and SCCA solo - I ran a set on my 128i and was amazed. I've driven cars with Motons and Penskes in the past and these are right up there.
Standard BC coilovers aren't awful, but I wouldn't call them an upgrade on these cars, either. The factory suspension is light years ahead of what's on most street cars.
RedShifts are around $1800 with Swift Springs. Even with 400/700 springs on my 128i, the car felt like a stock M2 on the street due to the excellent valving.
The single adjustable are the sweet spot. They've earned lots of trophies at Nationals for both SCCA autocross and TT. Also has excellent support, he'll answer any question you have and even do a revalve if you aren't happy.