Hi,
Yes on track under heavy breaking I found I could actually get the rear brakes to lock as the rear end went light, it was a little uncomfortable at times. No where near as stable as our FTO was around Silverstone which given it weighs in at about 1170kgs so about 65kgs heavier than an Abarth, & is an equally front heavy car running 324x32mm Nissan 350Z discs and Mitsubishi GTO 4 pot callipers on the front never once did this, and its brakes are mighty compared to Abarth brakes which feel strong on road but a bit weak on track.
To counter the understeer, we eventaully dialled in 10min total toe out at the front which is not far off the Trofeo cars. The one thing the Trofeo cars do have in 3mm (yes you read that right) 3mm each side of toe in on the rear to help stability and to stop the cars swapping ends, though they still do that rather alarmingly compared to other tin top race cars, this also helps the tyres heat up.
The rear can only be adjusted by way of shims, which unlike Clio's through Renaulsport UK or any number of retailers who have copied them, you can't buy from Abarth. If someone knew what they were doing it would be easy to just put washers in the rear to adjust things, in fact I know at least one Trofeo team did this to even out each side as the tolerance from factory is huge. I know someone is running parralel on the rear of his 500, but I thik this will make the back end more mobile which yes will reduce understeer, but also increase chances of swapping ends under heavy breaking. A little toe in which I think is factory anyway will help, and a little negative camber would to, I don't know off hand what it has on the rear, but for example my Clio from factory runs 1degree30mins negative on the rear with about 10mins toe in.
On a SS car, I would remove the bump stops to help rear end stability, and get a full geomotry check as from factory Fiats tolerances are too big and you can in fact get one wheel toeing out and one in and still be within linits.
Different tyres may help, Uniroyal Rainsport2 are very good, Michelin Pilot are good but PE2 and PS2 now discontinued and replaced with PS3, as are Godyear Eagle and new ContiSport 3 is meant to be excellent, never really had a great Pirelli on any car I've tried with them.
I have rambled on for a bit but hopefully some info hidden in there will help.