[RoundelM3]

OK, let's clarify things a bit here - "back pressure" is resistance to normal exhaust flow, caused by a more restrictive exhaust tract. It is NOT the pressure of the exhaust coming from the engine. If anything, the new muffler has increased the flow, but that would not cause any kind of lag and is not "pulling some flow" from the turbo. All the exhaust flow goes through the turbine side when the wastegate is fully closed, When the throttle is backed off (such as when shifting gears), built-up pressure in the compressor side is relieved by a valve that routes it back into the pre-turbo intake tract so as not to stall the compressor. If the function of this valve isn't working correctly or if the wastegate is opening (thereby reducing compressor speed), lag would be induced until the turbine can get the compressor back up to speed. Check those items, and also check the electronic connector to the valve on the muffler that opens/closes the passenger-side pipe and make sure that side is opening. So no, you're not diagnosing it correctly and no, increased boost isn't going to solve the problem.