11000ft driver

it is NOT turbo btw

it went down to 8.5 because of the increase in thickness to sti gaskets (I think) obviously at my altitude, a loss of compression was the last thing that I wanted, but the shop that did the work didn't mention that I would lose t with this setup (I guess it should have been obvious, but it didn't cross either of our minds) would 1/2 lb loss cause this huge change in efficiency?

I put a 2.5 sti short block in a legacy GT2.5, used sti gaskets, but stock heads. I'm getting ok power (comparable to what you'd expect from a healthy ej22), low compression, & HORRIBLE mileage) the engine runs PERFECT, so I'm leery about major changes, but need to figure this out. I drive over 11,000 ft twice a day, so the gas is killing me. any suggestions? I've got a weaponR intake & lightened flywheel, other than that no extras.