The ZL1 has an LSA, not an LS9 according to various sources.
In addition, the GT500 has a 2.3L Supercharger with a 5.8L motor. The ZL1 has a 1.9L Supercharger with a 6.2L Engine. Same amount of displacement when matching up both the engine size and Supercharger size.
Also, the GT500 runs 15PSI where as the ZL1 runs 9 PSI (Some say 10PSI max on the stock setup) so, there really is not a big difference between both cars. The ZL1 has cubes and is a Pushrod vs. a smaller Modular motor.
ZL1 also has a 9:0:1 compression ratio vs. the GT500's 8:5:1. As we know, the higher the compression, the less boost that is needed to make power.
Granted, the GT500 makes more power, but, as LOWDOWN mentioned, Ford doesn't have anything that beats Chevy HP vs. HP in stock for with equal displacement, but, the engines with blowers are the exact same size and the GT500 makes over 80 more HP.
Let's not start comparing the number of valves, VVTC, V-Tec, etc.
We are talking Pushrod vs. Modular with equal displacement.
I found this on C5.
http://www.camaro5.com/forums/showthread.php?t=300151
So, with a smaller pulley running 13-14 PSI, the ZL1 puts down roughly 560-570 RWHP. That is still less power than a STOCK GT500 makes with a 2PSI difference.
See my point?