Ken,
I have been using Revit with Parallels for a couple of years now. This year more so than in the first year. My hardware comprises a 2010 27" iMac quad core i5, 8GB RAM, ATI Radeon HD 5750, 1GB VRAM. I am running Revit Architecture 2011 on Windows 7 32-bit but I will move soon to Windows 7 64-bit as the 32-bit version is limited in how much memory it can use (i.e. <4GB). The VM is configured to use 2 cores and 4GB of RAM (no point assigning more with 32-bit Windows as it won't be used). The VM is set to provide maximum Windows performance, the idea being that if I'm running the VM that means that I'm doing mostly Revit work. When dealing with a complex Revit model or when rendering I tend to assign more cores to the VM.
Generally I am very satisfied with this setup as it means that I never have to leave my Mac. I am able to run Photoshop and InDesign natively while running Revit and there is no noticeable loss of responsiveness on the Mac. Revit does not run as smoothly in the VM as it does natively on a Windows PC. I believe that this is mostly due to the way that the gfx hardware is exposed to the host OS by the VM.
I would not recommend creating a boot camp partition. I don't believe that the performance gains justify it and you will lose a lot of the advantages of having a VM. For example, I have create my VM with just Windows and Revit on it. As I add more patches, plugins etc, I tend to make a copy of the whole VM first and then add new features to the copy. This way if anything goes wrong I simply delete the VM file and go back to the previously working VM (this manual procedure is slightly different from Parallel's snapshotting function but I prefer it as it gives me more flexibility). I also have other VMs, all configured differently with different software—when it comes to Windows, less is definitely better.
To measure your performance just time a complex render (tell Revit not to close the render window). Do it once on a Windows PC and then again on your Mac. Assuming that both are comparable machines this simple test will give you an idea of the performance hit imposed by the VM. I don't believe that it's much, especially for complex renders.
Also, Windows 7 has a built-in benchmarking tool. Right click the "Computer" icon on the desktop and select "properties".
In the dialog, under the "System" section click the "this system has not been rated" link (or something like that). Windows will then display new dialog giving you the option to rate your system. It tests CPU, memory, graphics, gaming graphics and disk performance. The scale is from 1.0 to 7.9 (7.9? What the hell? You gotta love Microsoft). Anyway, my system scores are:
CPU: 7.3
Memory: 5.5
Graphics: 6.0
Gaming Graphics: 6.0
Disk: 5.9
As you can see disk and memory are the lowest scores as is to be expected when running a VM.
cheers,
-tomek