A transition to OSX is a personal decision that only you can make, of course. Your specific needs are going to be different than someone else’s. What matters most to you about performance may be very different than what matters most to someone else.
I’ve been making the transition over to OSX and for me, it’s been worth it, although the transition hasn’t been without its bumps and potholes in the road here and there. But I’m very happy with my decision and I don’t expect I’d switch back to Windows as my primary OS for music/sound unless Apple royally screws things up. (Crossing fingers.)
There are many reasons why I chose to switch, and the plus side outweighs the negative side for me. Your process at arriving at a decision will be different than mine. Some people will criticize a move to Macs as an irrational move because, as some of the critics will say, Macs cost more and perform less than similar Windows-based machines. But as many people who have switched to Macs will tell you, performance per dollar is only one part of the puzzle, and there are many other factors that came into play that are meaningful to those who have switched.
And the fact is, Windows machines will indeed give you more bang for the buck. No bones about it. I don’t expect that price/performance gap to close any time soon simply because of one obvious fact: Macs are made only by Apple (unless you want to try the DIY Hackintosh route, which is a valid approach for many people). Windows machines, on the other hand, by the very nature of a vastly larger and competitive market, will have many more options for computers and parts that will ultimately cost less than a single provider like Apple. So basic economics here, you can get more horsepower for less money on the Windows side, in general. There are a few areas where Macs may be performing better than Windows (i.e.: C7.5 loading times appear to be one area, although I haven’t had a chance to confirm that). But in general, you’ll get more bang for the buck from Windows overall. In my own tests, I’ve seen about a 20-30% performance penalty for running OSX for my test sessions. And when you need very low latency performance (64 sample latency and lower, for example), and you are using best-in-class drivers like RME, a well-tuned Windows DAW will perform even better for those scenarios. Your performance for your specific apps and plugins (and don’t forget audio device!) will be different than mine. You may do better or worse, and only you can really find out based on your specific project needs and hardware configuration. There is no hard number, since all of our situations are different.
For me, that rough 20-30% performance hit is not that big of a deal for the types of projects I run, due to the amount of CPU headroom I have on my computers, and the fact that I also have VE Pro available when needed, which I can run on some Windows machines as needed. So for me, the performance hit is irrelevant, and other much more important things matter. Personal, project and client workflow trumps the performance hit in my case.
Additionally, the general issue of performance is becoming less and less important in my view, with current gen computers. I think it’s now irrelevant for most users, but there are people who will strongly disagree with me. I think you should run what you want to run, and you’ll do fine on either platform. Again, others will disagree with me. A current-gen Mac will of course be able to handle very complex audio production needs, so you may never even run into performance issues. And thanks to great programs like VE Pro, you can build inexpensive satellite slave machines (running Windows for example) to offload any extra needs… not to mention if you are going the UAD route, then you’re offloading DSP needs onto the UAD chips anyway… So you may never notice performance issues at all.
So the larger question goes back to whether or not YOUR specific applications and needs will have a performance penalty if you use a Mac, and whether or not you will even notice. If you are running complex orchestral scoring templates with massive sample libraries where you are already dealing with VE Pro, for example, then you may value performance much higher than someone who is running other types of sessions. For me, the current gen of Core-i7 CPUs has enough horsepower to handle most of the types of projects I do quite well, so it’s not an issue for me. But who knows? In six months, someone could release the latest killer plugin that I HAVE to buy, and maybe it will sink even the most powerful Core-i7 machines. It’s bound to happen at some point. U-he Diva on Divine mode will definitely hit your CPU hard. 
But take some time to really examine the types of projects you do, and that will really help you decide.
Again, as mentioned above, I don’t expect the performance gap will even out any time soon… mainly because of the scale of economics of how Mac computers are single-vendor and Windows are infinite-vendor with a far larger overall marketshare. While there may be as many actual Mac DAW users as Windows DAW users for various DAW apps individually (i.e.: I know far more studios running Pro Tools on Macs than Windows), the simple availability of far more hardware options on Windows will likely keep this price/performance gap in favor of Windows for the foreseeable future in my opinion. But again, that may or may not matter to you based on the types of projects you run.
Best of luck in your decisions!!!