I've used Pagid RS19 with ATE fluid and DYI cooling ducts into the wheel well at VIR full. Im on stock suspension and 265/285 RS3's, running at the edge between intermediate and advanced groups. Works fine, mostly - but I still managed the brakes a bit.
Let me explain. I experimented a few sessions and ran progressively harder on the back straight, braking later and later until I went past the last brake marker, quite at my fear limit and when the car on stock suspension was getting squirrelly. I could do perhaps 2 such maximum attack laps, after which the brakes got a little soft, and I backed off a little.
To me personally, it is worth more to keep the brakes stock with very good pads, cheap cooling ducts and a little bit of brake management. I also went with 18" track rims. Mentally, I cannot justify any modification costing 10% of the price of the car or more.
But if you want the absolute best lap times, and want to scare Boss Mustangs away on braking, you will probably need a BBK.
2009 E90 M3 ED
2014 X1 28i