IMO it's not worth the trouble.
SONAR and third-party plugins don't take up that much space, and disk performance isn't an issue, so the benefits are small. It's just samples that take up a lot of storage and I/O bandwidth, so there are plenty of reasons to site them elsewhere.
The issue isn't so much with SONAR itself, but rather installers for third-party utilities and plugins. They nearly always default to C:, and some make it unnecessarily difficult to switch to another drive. It's so much easier and less error-prone to simply accept their default pathnames every time.
Yes, you can use redirection as a kludge. But it really is a kludge, because regardless of what type of redirection you employ there is always the possibility of losing your mapping. Even when Microsoft itself does it, problems eventually ensue.