2004 Chevrolet Impala bank 1 lean

I have an Impala SS 2004, with a bank 1 lean issue. The car has been in the shop for a pretty extensive over haul. Replaced tranny, changed oil gasket, changed 2 O2 sensors, changed plugs and plug wires, change EGR valve, changed vac. hoses at the top of the engine and change the valve that connects the engine to the vac. hoses and brake unit (that round thing on the fire wall). The MAF was changed last year, and I don't think the car has really run correctly since then. ALL PARTS were changed with GM parts. The car does have an after market exhaust, system but it was made for the car.