Figured I'd jump in here when i read that our STB was making your car understeer!
Everyone here has already pretty much nailed it, but the car always understeered, you're just more willing to push it to that limit. Front engine, rear wheel drive "performance cars" are almost always set up to default to understeer as it is the perceived "safer condition" compared to oversteer.
And the change that you're sensing (despite what everyone here who maintains the "strut bars do nothing mentality despite having not done the testing and not owning one, let alone ours) is that the strut tower bar really is keeping the tension and compression between the two towers tied together. No longer is one towers going left and the other right over a bump or through a turn, they both are opposing the forces and either going one direction, or none at all. The most surprising thing I learned was in a turn (we posted all this data in our development thread) was that the loading on the strut towers was cyclic, that is alternating between tension and compression. Granted these are very small deflections, but when those deflections are going the opposite directions of each other they double. When They are tied together they don't anymore, and end up with deflections in the same direction. This lends to more predictable handling because the car isn't gaining and losing camber and more importantly toe on opposing wheels when going through a turn.
So, there are some good suggestions in this thread (and don't worry about camber wear if your toe is correct, toe is the tire killer). But i mainly chimed in to remind everyone about the rear suspension design and rear strut bars:
A front strut bar is effective because there is no material between the two front strut towers. Also the strut is connected directly to the spindle and thus wheel in a very stiff, solid connection.
The rear strut towers have a lot of material between them. Some obvious and some non-obvious. But the springs and shocks are connected to the lower control arms, and hinged. So the strut tower bar would see 0 to near-0 lateral load, and very very little "up and down" load.
So I'd put your money elsewhere too
Chase
Engineering