I have a 2005 GT3 with FabSpeed mufflers,crankcase vent, RS Lwf and clutch and Euro gears. I can't really give an estimate of the HP. The car runs strong. I have about 410 HP.
I am toying with the idea of a new 997 GT3, stock. Do you think the cars would be somewhat even in the 1/4 mil, 40-120 mph etc. My car has excellent throttle response from 40 mph up.

 

This link is from one road test with the 997 GT3.

The GT3 in the test was running sticky MPSC 235/305. We know they offer better grip than the MPS2.

The author references that the GT3 front end gets jerky on bumpy roads. This has been reported by 996 GT3RS owners as well.


2007 911 GT3

Weight: 3220 lbs (without sunroof)
Engine: 3.6.L
Power: 415Hp
0-99.4mph: 9.0 secs (0.1-0.2 secs faster than the 996 GT3)
1/4 mile: 12.20 sec (same or slower than the 996 GT3)

997 GT3 vs. 9ff Cayman 4.1L

This is an Euro test. Euro cars are lighter than U.S. cars. The GT3 was 110 lbs heavier than the 9ff Cayman, even though the 9ff Cayman had a bigger engine than the stock Cayman. The published weight for the 997 GT3 was 145 lbs higher than the published by Porsche (3075 lbs).
The published minimum weight for the 996 GT3 was 3043 lbs. My 996 GT3 weighed 3133 lbs brand new.

 

Hard to tell, but I'd think your modded car should do well in acceleration comparisons with a new stock 997 GT3. A track contest might be different given equal drivers, but this can be mitigated with the appropriate mods. We'll need to wait until the new cars start showing up. I for one, like the more spartan nature of the 996 car. It seems to be the last of the old school breed of 911. Currently doing my cup gears and LSD.

 

You should take the GT3 to the track and find out where it traps.