But my need for speed is more than my desire to make this car perfect,
Exactly the same thing happened to me. I had a fabulous Lux, it was almost perfect in every way, aside a cosmetic dent in the front wing. With just a little more effort (and the dent sorted) it could have been a concourse car.
All was rosey until I followed a 944 Turbo to Leeds for an event. The difference in power was spectacular and my previously much loved Lux was then inadequate. I had to have a Turbo and after that I didn't want to do anything with the Lux, suddenly it was just spending good money after bad.
So I bought a Turbo and sold the Lux. The Lux was in far better condition and, as with you, I had spent on everything that was, or might have, gone wrong.
At the end of the day however, at road legal speeds the Turbo is little better. Obviously the Turbo it is faster, though in trueth I've only seen 120 out of the Turbo but 125 out of the Lux. The Turbo is quicker, but not to 30 mph because of lag an a notchy gearbox.
So I got rid of an excellent car and bought one which is adequate. OK so the Turbo does give me a grin when it goes whoosh and, when I didn't necessarily have the power to stop a warm hatch pushing me on a duel carrigeway in the Lux (well up to 100 anyway), the Turbo makes them look very very silly. But I still miss my Lux.
Several I know have both Turbos and Lux or S's and enjoy both. I couldn't accomodate or afford both but I still miss my Lux.
The moral of this story? The Lux is a very, very good car. If you have a nice one try to stick with it, the S2 might not be that much better in real world (legal) conditions and many not be in as good nick.