Alot of work and money, but think about it, is it really worth all that? Is it going to make it go faster? Will it really look better? Is it that much of a change? I'm far from a stocker but I think it would be too much work, not as stable, and cost alot of money and time, and in the end it wouldnt be worth as much. If I were going to try to modify the roof for aero, I would lower it, but then again, that would equal less cabin space and I'm a tall guy. If you use the right/make the right body kit, you can achieve a more modern look with less extensive modifications. Removing the drip rails smooth off the roof for a more modern look. Painting the trim black or blending it with the body and painting it the same color as the car can achieve a more modern look.
In the end, You dont really have to extensively cut the old z up to achieve a modern look, this car was before its time.
Both stock roof's look to be the same height, so again I dont think it would be worth it.
Heres a lowered z roof... imo it doesnt look much better, and its probably now a bare to get in and out of.