At cruising your AFR should be at 14.7:1 regardless of your mods. The narrowband will always maintain 14.7:1 to keep a healthy cat.

Lowering the boost does lean out the car at WOT since the car hits lower load cells in the fuel table and these cells are leaner than the higher load cells. The downside is that the car will hit higher timing cells as well and that might cause knock. Your best bet is to get your car tuned. Go to socalevo.net and check out who is tuning the Evo X. There are many of them.

 

I drove around for two weeks with all my mods and no tune and it was a night and day difference in performance and fuel economy. Tune FTW.

Chuck the MBC and go with ECU controlled boost on the X. Fuel pump not required with your current mods. The Evo X pump is very good. I am tuned at 26psi with the stock BCS and fuel pump.