I have driven both 2.0 and GTA on track.
Yes, the 2.0 is more balanced and more nimble, but that only goes so far. The extra horsepower and torque of the GTA more than makes up for it
For your car: Yes, Eibach antiroll bars are a must. Best suspension upgrade on the 156, ever.
Coilovers.. well a lot of track folks swear by the Intrax HDA kits, so they must be good. I havent tried them myself, though. (Still havent bought any coilovers
For brakes, the GTA setup of 305 / 330 is great, but most importantly, get good brake pads and fluid.
You already have the diff, so well done on that.
Engine tuning is going to be limited for a 2.5 What you really want is to get a 3.0 or 3.2 engine. No point in spending money on engine tuning for a 2.5 really.
And apart from that, weight saving... add all the lightness that you can to the car.
And have fun with it!