I went from 8.4:1 down to about 7.5:1 on my 7M. I didn't notice ANY difference in off boost power or lag. If octane wasn't an issue for me I might of run 8.0-8.4:1, but I like to make as much power as I can on pump gas. I can run about 20 psi on 91 octane with out H2O, 24-25 psi with H2O, up from about 14 psi with the 8.4:1.
Stock L28T pistons can take 20+ psi all day long as long as you don't detonate, I do it everytime I roadrace with cast pistons, 1/2 hour at a time.
Electramotives numbers aren't anything magical. Port a head, more cam duration and rev it to 7500 rpm. Electamotives would be about 470 rwhp. That kind of power could be made with 7.4 if the head flowed and the turbo was up to it, just like theres was. 20 psi from one turbo may only make 300 (stock or upgraded T3) while from another it will make 500 hp.
Dapiper on this board made 450rwhp @ 20 psi, don't know his compression though but not far of from E motive made.
The size of the turbo has to match the power you want to make. If it does more boost will = more HP. Boost and rpms are the only way to make more power form a given displacement. A T3/4 won't get the job done if power is your goal, it doesn't matter how big that compressor is it's the T3 side that will limit you. It will spool fast at 2500 rpms which seems to be important for most, that is until they realize they only make 250 rwhp at 15 psi, but atleast it spools fast.