To answer your question directly: yes, Mono's performance is fast enough for developing on the Mac. When we ported Remoting code from .NET to Mono a few years ago, things went very very badly but I've been told that Mono has been greatly improved since then. Regarding development environments: #develop and MonoDevelop are cross-platform.
Now to get practical:
The gist behind this question seems to be the idea of avoiding purchasing a Windows license and using Mac and Mono only. This is a terrible idea. I would never release any product, even internally, that I've never even run myself in the intended environment. Even if you abhor Windows and all that it stands for, if you're writing software for Windows, you owe it to your users to thoroughly test it in Windows.
That said: I'd recommend Boot Camp and Parallels/VMWare.
Boot Camp
The best .NET development tools are Windows only. Even if you find an alternate dev. environment that you prefer, you'll eventually want to test your apps running at native speed.
VMs
VMs are good enough for most testing and debugging. If your app has the possibility to do any damage, you'll especially want to use VMs for sandboxing. At my last job, we did low-level invasive scanning and tweaking. When our code was buggy, we could REALLY mess things up. All of our testing happened in VMs long before we put them on real boxes.
The above will allow you to own only a Mac (assuming it's Intel-based) and will still allow you to do all of your development in OS X + Mono if you so desire. It just will not allow you to avoid Windows.