Well, exhaust velocity must be high, as there is a large volume of gas being pumped through the engine. It needs to go somewhere, and reversion doesn't account for THAT much...

I would guess that you will see WAY less than a tenth of a second delay in WB readings. In a 3rd gear pull, that is not too many RPM. Not enough to really affect tuning, I don't think.

This is pure conjecture, but I started with the principle: where does the WB probe go when you go to a dyno?