Finally got around to changing the oil...it was "only" -12C tonight...
Anyways, the stock filter that came with my '05 2.5RS wagon is indeed the small filter (A100) that is now used in the WRX, per Sticky above, not the A060 filter.
The filter from the dealership (SCI530009) is the same size but the construction of the Japanese filter looks to be better (seal has round x-section vs the typical square). I picked up a filter for my Passat 1.8T also since the dealerships were next to each other. VW went the opposite direction in terms of filter size, after problems with oil sludge clogging things up.
Any wonder I was nervous after getting that tiny filter for my 2.5RS?