Was looking into this myself... lots of controversy out there...
Cans seem to be OK if it's just for venting the heads... crank case seems to be a bit more of an issue. Sorry if the following is a teaching-granny-to-suck-eggs job but hopefully it's useful:
When cruising (ie manifold under vaccuum/ not boosting) there's a valve (PCV system - plenty info on google, not just scoobys) under the throttle body that sucks the gases out of the crank case. Under boost, the gases are sucked into the turbo inlet... in both cases that means the engine ingesting a (hopefully but no necessarily) small amount of oily air - clearly this aint great for knock resistance!
However, trying to vent the crank case is useful as it improves piston ring seal assuming sufficient suction/ vaccuum can be acheived... clearly a catch can is kinda passive so this ain't gonna work for the crank case. Better than nothing re oil catching perhaps, but far from perfect.
Air-Oil separators seem to be popular, especially those than plump onto the oil filler cap (Pro-R/ Tomioka, Prova, Grimmspeed etc) - they have a classy, stock look to them, but again they don't fully solve the crank venting issue. Crawford Performance's one seems well engineered but it's not really any better by the sounds of things.
What does work? Dry sump anyone?
Bottom line - catch can fine for heads... less so for crank case... and as ever, better solutions aint cheap!