OK, I think i've found a solution. It's not great but it'll do.

Things based on trace aren't ideal, as I want to stick the results in a db, and i don't want to interfere with the normal behaviour too much.

For the RESTful ones I'm going to start using length($response) (thanks for the tip on Content-Length, AltBlue)

For the SOAP, I have two possibilities. I can either use outputxml and parse the SOAP response myself, or I can re-serialize the SOAP::Data response and just do a length() on the results.

I think I'll do the latter. It'll be inaccurate, but it's ok as long as I'm consistent, since I'm trying to get results for the feeds relative to each other.

Ian