LQ4 runs a 7cc dish piston whereas the LQ9 and L76 run a flat top piston. The dish drops the compression of a 6.0L with L92 heads to 9.6:1. If you want higher compression, you can flat mill the head or run a thinner gasket.
You could run an LS6 cam with L92 heads and make over 400 flywheel horsepower with ease assuming you have an L76 intake and headers. This would be a great 87 octane motor.