Don't think you'll find much since Q4s did not sell in sufficient numbers to justify uprated anti-roll bar development programs. I know that Eibach do all 155s except Q4. It might be worth trying H&R, OMP, not too sure if Sparco does anything..
TAZ is right. But, you can do as Steve Webb did: Fit a Lancia Dedra Integrale anti roll bar front. Then I do think the rear one looks the same on all 155's. I'm not sure but it looks so. Otherwise I don't think it is so very expensive to get them made after your own requests. Perhaps worth a try?
If you use standrd shocks, start with an upgrade of them and then reconsider the anti roll bar issue.
Andreas, what shocks do you recommend? If I'm taking the plunge for new springs & dampers, I fancy 2" coil-overs. what are you running. Also, did you ever get those anti roll bar mounts made, I remember a while back you were going to rose joint the bar on the front..
I use yellow Koni (adjustable); quite cheap, good and the only ones available for the Q4. You can get Öhlins from Japan for the price of 2500 Euro and I am sure they will be calibrated for the 155 model you want ...
... only problem, that I realised last thursday after having demounted discs, calipers and wishbones, was that the only part of the anti roll bar that can't be seen before the operation, i.e. the rod inside the drop link bushing, is 12 mm in diameter. Not 10 as everything else. What I say is that the upper unibal is too small Did I sleep much that night??????? Nooooo!
But did my Q4 run extremely well on the trackday yesterday anyway? Yes!!!