It's an awesome street/track combination car, which by definition is already a serious compromise. As many have said if a second a lap or two matters, and you're as good a driver as you can be - then a 3600+ lb car was the wrong choice for lap times anyway.
If you like the moonroof for its utility don't hesitate - I didn't. Of course I chose a Gt3 over a GT3Rs because I want to have the option of cruising with sun in the cabin....
You have to enjoy the ride.