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some good recent threads on the rennlist 996 forum on this. basically, if you replace the aos with a catchcan (or an aos with a higher-flow diaphragm) then you introduce too much crankcase vapour during high vacuum idle situations (ie when you come off throttle after wot) and screw up idle fuel trims.
if you put a catch can after the aos then it has to be of sufficient size/volume to make a difference should the aos fail.
one solution that seems to be effective is to vent the aos to the airbox. i have yet to research it to any degree (i am of the 'just use the most recent version of the aos and you will be fine' school) but apparently it doesnt foul the air filter too much, and the airbox is of sufficient volume to hold the oil should the aos puke up. not certain if there is sufficient vacuum at this location to positively vent the crankcase, or if this vacuum is even necessary.
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