Okay first, please suck on this lollipop while I answer your post. 🍭
Under my own testing on three different macs, I get equal performance to my PC, and can play osu! to the same level I can on windows (using wineskin). This does not mean I have stopped work towards a more native solution (by native I mean mono), but these things take time. Progress is being made, and in fact, the next big step is switching to .NET 4.0 targeting, which is ready at my end. I am simply waiting for enough users to upgrade their PCs to make this transition as seamless as possible.
I would love to know what kind of mac you are attempting to run osu! on, and whether you have fiddled with the settings. I can't imagine how your performance can be so shocking you would make the statements you are.
Notes:
* "sky-painting" took 30 minutes.
* i have spent many hours working towards .NET 4 targeting. no, you haven't noticed this because you don't read the changelogs, follow my dev streams, read my twitter or blog, etc.
* i haven't been able to actually make forward progress with osu1 coding because i've been too busy keeping the game running. yes, that takes time and effort and most of that is handled by me alone.
and yes, as you say, gaming on OS X will always be sub-par. the operating system itself doesn't lend to 3d gaming (it hardly handles 2d desktop rendering and multitasking, which is why i'm gradually moving away from it to run windows across all my PCs/notebooks).