If you do get lighter rims, I'll be interested in how it affects the ride and handling, and the basic feel of the car. Of course the performance would increase, but.... I've assumed that the weight of the car in general, and of the wheels in particular, are a big part of the wonderful feeling of stability and solidity in the TSX. I'd never sacrifice those things for the extra performance, even though I know the increase might be considerable.
Changing wheel weight significantly can adversely affect suspension performance. In fact, reducing weight can make the car handle worse in some cases. Clearly, reducing unsprung mass is ideal, BUT the OEM suspension settings (the relationship between the spring rate and damping rate) is tuned to the unsprung weight of the suspension. Changing one without tweaking at least another can result in poor performance. That's the idea behind adjustable dampers: you should set them according to the new resonance characteristics of the suspension.