I'd go for the GTA bars, but make sure you do front and rear to keep the balance.
If you just change the front it will make it understeer more.
I have concerns about the balance of the Eibach ARBs for a 156 SW 1.9 JTD16V. Those bars were for sure developed for optimal performance for a GTA which is more heavy at the front. The Eibach ARBs have 4mm more both at front and rear than the original ones. Isnt 26mm too much for a 1.9 JTD. I want less roll but also want the car to understeer less (which i already worked on raising the front tyres pressure a bit to decrease understeer, but i prefer to do this the right way with some arbs).
I'm with Bilstein B12 Pro Kit suspension and as some probably already know this suspension change the rake angle of the car. The rear gets a different front / rear height ratio than the original suspension. With B12 the rear gets lower in comparison to the front and that means that there is more weight on the rear, making the front lighter and of course making the rear more planted but the front gets more understeer. I want to reverse this effect a bit, so i'm worried that keeping the F/R stiffness ratio wouldnt work so well on a 1.9JTD due to the engine being lighter than the one on the GTA.
In the case of installing GTA ARBs the front as a 1mm improve and the rear 2mm improve meaning that the rear will get stiffer, which also means that the car will get more oversteer in comparison to the original setup.
Which are your thoughts about these variables?