I have 18's on my car currently. I have a spare set of 19" ADV.1's for my 996TT. I mounted them up once on my 930 to see what they looked like out of curiousity. MY particular set has a large step lip setup and they honestly don't look like 19's. If my car was lower, the 19's wouldn't look horribly out of place.
With all that to say - I think it depends more on the style of the wheel. a 19" monobloc wheel with no lip looks a lot bigger than a 19" step lip.
For reference, this is my 930 on 18" ADV.1 5RS's:
This is a shot of my ADV.1 10TF's on my 996TT:
I guess it's hard to imagine the wheels flip-flopped, but the step lips on the 19's make them look smaller. Honestly they didn't look as bad as I would have thought.
I wouldn't hesitate to run a 18" wheel on a 930.
Hope that helps.
Chris.