I've bought - but not yet fitted the Alfaholics rear ARB for my twinnie. I'm waiting until I've managed to poly bush a fiat coupe front ARB to go on at the same time.
The Alfaholics rear ARB is 25mm diameter, which I reckon makes it 3 x more resistant to roll than the stock bar. The 23mm coupe bar should increase front roll resistance by roughly an extra 75%. As a note, the Ultra racing rear bar is 20mm diameter, which is not a lot more than standard.
I asked Andrew Banks at Alfaholics about the dynamics of this bar and his reply was:
"The funny thing about suspension setup is that it's easy to theorise about what might or might not work or what the effects might be but at the end of the day we find that there's no substitute for just going out there and trying different things. We made loads of front and rear bars and tried dozens of combinations. We confirmed our initial thoughts very quickly that GTVs are far too soft in roll as standard so we could increase bar diameters quite considerably. However, it was also easy to get the front bar too stiff and induce excessive power understeer. This didn't appear to be such an issue at the rear, because the GTV is inherently an understeering car and we could never make it snap oversteer - just drift in the wet when you lift off. We found that 25mm gave the nicest balance and feel to the car both in initial turn in and behaviour on the limit. We drilled several holes in the end of the bar to make it adjustable although the differences are fairly subtle. I wanted to be able to tone down the effect of the rear of the car coming into play on the limit for nervous drivers. The reality is that the car will never do anything nasty, even on the stiffest setting, so it's just about driver perception. The most important thing is to get the rear roll stiffness essentially correct and we found 25mm to do just that."
Hope this helps