I've actually had the opposite experience. I work on Firefox support for a very heavy Angular2 app, and I usually see 2-3x faster reflows on OS X (and Windows) than on Linux, and faster paints too (although I haven't taken exact numbers, since reflows are the bottleneck). It seems that at least the graphics pipeline is much better optimized for OS X/Windows than Linux.
I don't know whether it has accelerated compositing, but in my experience compositing is relatively cheap compared to painting the layers or especially layout. That may be an artifact of the app I work on being poorly composited, however.
