You are right the Q2 is not a plate diff, the Q2 is definitely a torque sensing diff made by Torsen, they are a competitor to Quaife who make a different brand of torque sensing diffs.
for more details, apparently the Q2 is a Torsen T-2 diff.
Torsen diffs are a little nicer on the road than plate diffs and also zero maintenance. Plate diffs are a little more hard-core, slightly faster, slightly noisier, and more maintenance but adjustable settings.
I have driven a stock 2.4, and yes the brakes were not great, the problem was at least partially to do with feel as well as stopping power. I found on all the 156 range (except the GTA), you really had to stand on the pedal.
Apparently the T-spark 2.0 stopped from 60-mph in 2.6 seconds which objectively is quite good, but honestly it felt crap on the road..
The 330mm GTA setup is rather good though, pulling > 1.1 negative g with decent pads and type-r tyres, I imagine your tarox 6 pots should be certainly up to the job.
You will have to post an on track vid of one of your trackdays sometime