Seriously thinking of an iMac -- anyone know?

NO.

There is no Mac version of Sonar. Yet!

There has been an announcement of an alpha version for Mac OS to be released for testing but it is late in arriving and no further information on its development.

Even after the initial Alpha release, it will be some time before the Mac OS version has anywhere near the features of the Windows version. Keep in mind Alpha means 'not ready for prime-time' almost like an experimental or proof of concept version. There will be lots of things that have bugs and lots of features that will not be included at first.
Some things like DXi will probably never be supported (at least natively).
For all we know, the Mac OS version could be abandoned after the Alpha release.
I would certainly not change to Mac until well after the Alpha release.
If you need a new PC now, and want a Mac, then run the Windows version of Sonar under Bootcamp.

Thanks folks, that answers my question nicely!