Best way to handle this?

Ok so my Mazdaspeed 6 is having some issue. Had a bad turbo. Me and a friend replaced this bad turbo with a working turbo that was suppose to not be smoking. Supposedly had 10k miles on it. Car is still smoking. Broke the BC solenoid while the installation of new turbo so i bought a new one. The car just after maybe 2 month finally threw a CEL which turned to 3. P0138 - The Rear 02 sensor, P0234 - Overboost and P0480 for cooling fan 1 control circuit. I disconnected my boost gauge. The only mods that are done is Second Cat Delete, a CP-e Cold Air Intake and the Cobb AP at stage one. The smoking is occurring heavy only in the left muffler. and this sucker smokes hard. i can post a recording of WOT if needed. Seems to stutter a bit in WOT. Any ideas or if i should

A: Take to Mechanic and tell him to figure it out?.
B: Buy new turbo and tell him to replace mine. Then see if any problems occur after that.

I'd hate to tell him to find the problem and him not know about a possible turbo seal leak thus causing him to work for long periods of time on my car and not figure anything out but if it could be another problem other then the turbo then so be it. My coolant levels havent changed so i dont think its leaking coolant plus smoke doesnt look blue. Had my last oil change 2000 miles ago (still at about max) and will change it next time to a 5-40 oil see if it helps. I just want the smoke to go away! (unamused)

Any help would be most gratefully appreciated.
06' Mazdaspeed 6 GT
the turbo isn't smoking because its "bad" there is something to do with seals and back pressure and stock turbos I'm not 100% about the details but if you search you will find the answer its something along the lines of the reasons i just mentioned...