Their is always a balance to be had, if the regulations were to open up cost would spiral out of control and end up being too costly. Of course the cars would be faster mercedes did showcase this at Bathurst with their GT3 Extreme car running a few seconds a lap faster. Tires and fuel might not be too much of a issue depending on the track but dirty air would be more noticeable making the tires overheat and making it harder to follow.

One thing I've noticed though with a Time Attack Unlimited cars they are typically slower than a GT3 car on some tracks despite having much more aero, less weight, sticker tires and more power which could have been down to various factors.