As I understand it, the extension should give a tad more bottom end repsonse. I choose to not run it only because being in a MBX6, things are tight enough already. It would be tough to get it in, I think. As far as fuel, I've only ever run 30%. I would imagine if you're comfortable with your tuning skills, it would be fine to use 25%, only you might experience very slight temp increases. Ideally, the E.T mills are shimmed for 30% though, so you should also take that into consideration.