Need some advice on some cylinder head gaskets

I think changing the piston heads is another serious option.
I’m curious though. Naturally aspirated, this engine dyno’ed at 506 hp and 531 ft lbs of torque. Decreasing the compression ratio to about 8.5-9:1 what kind of power loss am I sacrificing?
My initial goal for this current project to get close to 850 hp to the flywheel.
Keep in mind that the only difference is the Holley 750 double pumper is gone now, and going multi-port injection, and obviously the piston heads if I were to change them.