I'm inclined to believe the tune-up and injectors are the problem, and not the new exhaust system.
If the fuel mixture is too rich, excessive carbon could making it past the converter, if you are smelling a gas odor.
The particulates are to small to notice just by looking at the exhaust being emitted from the pipe, but collectively, carbon deposits will eventually collect along the rear surfaces of the car (bumper, trunk, rear apron, etc.)
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car