I am setting-up a REST web service that just need to answer YES or NO, as fast as possible.
Designing a HEAD service seems the best way to do it but I would like to
You could easily make a small test to measure the performance yourself. I think the performance difference would be negligable, because if you're only returning 'Y' or 'N' in the body, it's a single extra byte appended to an already open stream.
I'd also go with GET since it's more correct. You're not supposed to return content in HTTP headers, only metadata.