I definitely think it's something specific to your car or driving habits. I have a very similar mod list to you (built motor,278 cams, 52lb/min turbo, 850cc injectors, etc)...
And I get about 15-20mpg city, 25-28 mpg HWY. Of course, this means driving the car like a responsible person and doing like 70-75 on the highway. If you start hot dogging around and getting into boost, that will quickly change to 10mpg.
If you want fuel economy, drive it like you would a fuel efficient car. I typically average about 275 - 300 miles per tank in mine - and that includes abusing it multiple times per fillup.
For the most part, mods make a motor more efficient. But when you start getting into the throttle, you're making a lot more power and using more fuel to get there. If you dont use the power, you wont use the fuel (unless something is broken).