Burning smell only when decelerating

[92_awdtsi]

So I've noticed this with both of my 92 tsi's. Only smells when I let off the gas and coast on the highway, the wideband shows it leans right out until I press on the gas again. Any ideas? One of my 1gs was stock and 1 is built wiseco/eagle bottom end.
Also, how much will a 2.5"exhaust impact performance of a larger turbo ex-35r compared to a 3"?

 

[dsmlifestyle]

The the fuel injectors shut off on decel until about 1500 rpm's on most FI cars, so the WB showing lean is normal. As far as the smell, I would check you PCV system.

The performance differences will show up at greater boost levels, since more boost=greater velocity=greater turbulence= more back pressure (head) on the turbine outlet.