Just out of curiosity i ran the C3 performance test quad issue with and without Families as well.
I can measure somewhere around a 30% hit when you add a family to the sprite being benchmarked. I would guess this is the polymorphism issue, which I've always suspected was an issue, but never found a way to isolate the overhead of - so this is an interesting test because you've found a way to highlight it!
The good news is... I've actually got enough of a working prototype of the C3 runtime to benchmark this already, and the C3 runtime does not have any performance impact when you add a family. So this is a nice confirmation that the new engine is indeed going in the right direction with its architecture, and should fix this problem throughout the engine. This could mean quite a big performance boost for large projects!