Ummm a vacuum leak from the crank case that builds pressure which is the whole purpose behind a breather ???? Several high boost engines use breathers as the rings do allow for some blowby and this allows it to relieve pressure in the crankcase and not recycle air by pushing it through the intake. I also belive that PCV systems are only effective at a certian level of crankcase pressure and designed for the stock levels.If all you do is replace the valve cover oil cap with a breather then you are introducing a vacuum leak into the engine. If you have a scanner or access to a Tech II, monitor your LTFT's, then pull the cap and check them again. You will see the LTFT's spike and probably over time get a check engine light. The PCV system uses engine vacuum to pull out contaminants, but it uses air after the MAF, so it is properly metered. With a breather you are introducing non-metered air, a vacuum leak.
Some racing programs actually use a vacuum system on the crankcase to cause less pumping effect in the crankcase and allow them to use lower tension rings thereby reducing friction.
The caustic approach may have some validity over very long periods of time.
If he puts a breather on his valve cover to vent the crankcase and caps his hump hose port, I can not see any way for a vacuum leak nor unmetered air. Also LTFT are just that Long Term Fuel Trims which are just a summary of counts of Short Term Fuel Trims which if his tune is dialed will show niether and will never register when in Powre Enrichment.
Just my $.02
P.S. Curtis if your worried about it, just watch your wideband when your in boost.