The breather setup has no relevance to perfomance or what boost you run at all. What it effects is where the crank case pressures go. Standard they go back to the metering unit, Bailey setup also back to metering unit, but you can vent to atmosphere on either setup and block off the metering unit return. This is a good idea on MFI setups because breather system air leaks are common and will effect the fuelling of your car if the flap is not being sucked down properly. Also who wants oily air going back into the engine and helping Det along it's way?!
If your breather pipes are blocked then you might get your Oil filler cap blow off of your dipstick pop out, which is another reason people ditch the old breatehr setup (you could just clean it though).

Karlos.