Changing the venturi was always a stupid design decision by EC. The entire air metering system is built around the flow characteristics of the venturi. That is a completely different issue and causes the same issue in practically every modern car unless you can change tables in the ECU correctly.
According to turbosmart...You lift off the gas, bov or recirc vale opens, at the same time engine load goes to zilch and you are in engine braking and that causes fuel to longer be injected at all.
You can't be rich or lean if no fuel is injected. Also a rich condition is not bad for a turbo car anyway...
Vent to atmosphere BOVs can also cause slight over-fueling between gear changes as a portion of metered air is released from the intake system. This over fueling is not harmful to the engine as the unburnt fuel will exit the engine as exhaust. If the slight cavitation and over fueling is unwanted, then most of the benefits of fitting our vent to atmosphere BOVs are in the plumb back but without the vent to atmosphere noise, slight cavitation and slight over-fueling.
And thats downplaying it from a bov manufacturer themselves.
You can get this condition on transient responses as well and where i also noticed it was goosing the car backing out of the garage where you are off idle and during cold start and fueling continues.
Rich like 11:1 is fine. 8:1 es no bueno.
Plus you lose out on the factory bpv keeping the compressor spinning. Borg Warner even incorporates this recirc function built right on the efr turbos both for performance and Borg Warner also specifically states this is "...to keep maf systems happy"