If I'm not mistaken the CCV mod is where the crank case vent (breather exit) is routed to some external point other than exiting into the turbo inlet tube. (on the 7.3's the CCV is on the left bank valve cover just under the turbo intake tobe)
I don't have it on my '01 250 yet but I can see some advantages - one being less oil mist ingested into the turbo and intake system (including the intercooler and the intake tubes). One disadvantage might be that the oil mist routed to an alternate external exit, would be thrown overboard and create an oily, dirty area where ever the exit was placed. A cure for that would be an air oil separator but even that would eventually have to be drained at some point. I've heard of some that just put it in to the exhaust pipe to presumably be vaporized.
Not sure if there is any performance gain to be realized or not.
Hope I'm correct on all this. There are threads on it some where in here.