Yea, it's definitely difficult to get more rubber down back there. Here's the breakdown the way I figured it:
ESS pros: solid tune, solid hardware
ESS cons: less power than VF
VF pros: solid hardware, more power than ESS
VF cons: bad tune, bad customer support
That being said... if you are really wanting an SC, I would recommend the following:
snag the VF570 from RatedM here on the forums $5000 shipped (bargain price and a regular here on the forums) -->
For Sale Here
snag the VF570 tune from Frank Smith ~$1200 (solid tune, has worked well for other zposters)
For a grand total of $6200 you'd have the most powerful supercharger you can get, solid tune, and solid hardware.
Whatever way you go, best of luck to you!