I think I was at 20-22 on pump gas. It'll be a lot better than what you have now. Even those of us that convert to E85 average 15mpg. The gas tank on the X isn't very big so the range is what, 260 miles?

As far as mods go, you can't get away with a whole lot if you for sure want a warranty. I'd say drop-in air filter, cat back exhaust, and a tune that you can revert to stock when you bring it in. The stock tune on the 2010's isn't bad but on a 2008 you'll want to fix it.

The X MR also has Bilsteins. The GSR is stiffer but I previously owned a 3-series and I've had sport-bikes so I expect a car like this to be taut.

 

I dunno, I would stick with the M5... it's an M5!

 

I tried all combinations. The Evo X GSR with KYBs is softer than a 05 MR with Bilsteins. And the Evo X with Bilsteins is softer than the X GSR.

 

The main restriction of the exhaust is the catalytic converter; cat back exhaust for me is pointless and noisy. Keep it stock if you won't remove your cath. there's some tuners that will make a few more HP from your evo with the stock exhaust.
Yes, stock is 20-22 mpg now that I remember driving like my grandma, floor it and that boost is going to eat your fuel. Still is more economic than any V8 charger, challenger, mustang, etc