u may need to tune the ecm. sounds like u have some excess fuel thats not getting burned up in the cylinder.

 

Here's my theory, with this engine anyway. The crank sensor controls the pulse signal to the injectors. Downshifting raises the engine's rpm's. Wouldn't that be pulsing the injectors faster as well? Or is fuel flow to the injectors cut back by the TPS? If the injectors are still squirting, then you're pouring some raw gas out the exhaust because the closed throttle blade creates a rich fuel/air mix. If the injectors don't keep pulsing, then it could be a momentary rich condition as soon as the throttle blade closes. And a more free-flowing exhaust will make that condition more evident.