I've got a site with a pair of 5500's and about 1000 hosts behind em and about 650 humans at that location. CPU's run quite cool even with ssl scanning+content filter+anti-malware on but load average has been an issue on one of them for the first time ever this week. Working with support on it but seems to involve something to do with caching, as if caching is turned off, the issue disappears entirely.
Tell me where i can find the metrics you seek in the interface i can find the metrics you seek, and I'll share.
I'm WAY happier with this pair of MWG's than I was with the piece of junk Bluecoat 510-20's (+ pair of AV510's) they're replacing.