If you make bank why are you worried about $4 a tank? Your saying you want to get 20 more miles on a a tank. That's less than 1 gallon. You might be able to get that just by pulling the spare tire ( you have road side assistance right?). And only filling up with 1/2 a tank... no need to drive 150 miles with 6-7 gallons of in needed weight.
Why 320 miles? Is that the number you need for a full work week with only 1 fill up?
If you want the rumble than your MPG will only be going down because you will use the gas pedal a lot more to hear that rumble... and its quite addicting.
2001 ZX3 2.0L Zetec-
Hatch Nation #42
Going through Rebuild See "The TICK Build Thread" for details (http://www.focusfanatics.com/forum/showthread.php?t=289168&referrerid=62897)