A 30cc engine is going to be struggling big time with a plane in the high teens, especially if you're looking for some semblance of sport flying. The 30's are just about done at 15 and 16lbs. I would say the Q41 and G38 aren't going to do much better.
Now if you're talking about a plane that's going to need weight to balance with a 30cc engine..... that would be all the excuse I would need to jump up into something maybe twice that size. That's me though.