The increase in PS was more likely to be down to not having hot oil vapour recirculating back through the combustion cycle, i.e. a heavy breathing engine.
I remember Andy Boakes racing in the VW Cup at Croft, lap times getting slower and slower lap by lap during practice. Removed the boost return after consultation with the race director and also Brian from Jabbasport who was there also, blanked 2nd inlet, back upto full speed again. Engine was shortly rebuilt after that though.