OP.... It sounds like you are more show than go anyways. Wanting 20" wheels and more power under 4000rpms and all.
If you are going to drive the M3 like a M3 then go for the S/C and enjoy the added power all the way to redline. There are people that will back each S/C company and I'm backing ESS as I had it on my E90 M3 and loved the added gobs of power. Both ESS and VF offer an upgrade option if you want more power later, but I'm not site about G-Power. However, look at the power that the ESS entry-level kit is putting down. You'd have to upgrade the VF entry level kit just to compete with the power the ESS-535 kit is putting down. Just sayin!
At any rate there are plenty of guys on this forum to set up some ride-alongs, so meet up and test em out!
Anyone with a S/C entry level kit can hook this member up with a ride-along?