SAN Performance Slow
After some more investigation, it turns out that the issue was someone (cough) enabling flow control on the SAN switches on the advice of an internet article talking about the very same hardware and explicitly encouraging Flow Control to be used.
After disabling flow control, Load Average and I/O Wait immediately reduced, and after 24 hours it is completely back to normal (0.0%wa and Load Average < 0.10) once some struggling maintenance tasks completed.