For me its usually, jack up rear/front of car, check all the suspension, bushings, mounts, exhaust, diff, brakes, usually anything I can touch and check for leaks/movement/noise cast my eye over etc. I'm no mechanic but have enough knowledge to know what I'm looking at, as well as just having fun doing so.
Diff snorkle is a good theory, but there isn't a lot of room due to the travel needed for the suspension. As doug mentioned there is the homologated larger/finned plate that might have more of a baffling to it. Or just keep it in check and top up as necessary.
Options:
http://www.rhdjapan.com/jdm-low/STI-...Rear-LSD-58657
http://www.perrinperformance.com/pro...ory=68&model=2
Anyone have pics of the inside of oem and sti plates for comparison?