Will,
You're quite right. Rear discs are standard on the TDi (90) and the 1.6 FSi.
I'd agree, having tried both, the difference feels marginal, even braking from 3 figure speeds.
The 1.6 FSi has a bigger rear spoiler but the 90 doesn't.
Shaf
Nov 2002 TDi Sport, Silver,Black/grey interior,climate. Replaced previous Mar 2001 TDi SE