Don't feel too bad, the 135i is often underestimated. Here is an idea of what you were up against:
http://www.burgertuning.com/jb4_pnp_BMW_performance_tuner.html
Sample timeslip at the track with only a JB4 tune:
https://youtu.be/LKp50GF3bYQ?t=1m44sN54, 6at, jb4 iso, dci, intercooler, downpipes, MT drag radials, 65% e85, 35% 102. Map 5@17-18psi.
11.92@123.6mph
Tune nets you approx 80 whp (they have varying tune levels also)
Intercooler can extract another 20-30 whp
Downpipes (catless) add 25 whp (when tuned)
If you haven't had the drivetrain fluids (trans/PTU/RDU) flushed recently, that's the first thing I'd pay attention to. The 135i has up to a 1000 pound weight advantage, cut down to 880 lb with 120 lb passenger, still making the mods worth more motivation/lb.
135i on E85/93 octane mix, JB4 tune, Raceland downpipes, and Helix FMIC
50/50 mix of E85/93 is mainly used as an octane booster and takes advantage of map 7 lightweight race map