I've used SRF and Motul RBF 600 You will likely need to bleed motul after every weekend or 2-3 days at the track. Your mushy pedal after some track use on Motul is normal.
I kept SRF in for about 9 months and it was still good. When I finally bled the SRF out, it was super dirty but still usable.
...but Motul is still cheaper over the long run if you bleed the brakes yourself. I go through about .5L every bleed.
If you pay a shop to bleed the brakes, SRF is the way to go.
2015 F34 328iX GT Glacier Silver
2011 E90 M3 Monte Carlo Blue
2004 E46 M3 Imola Red