I mentioned this morning that I was having slow SAN performance. I said that I'd post an update if I figured out what was wrong, and I did.
EMC AX4-5's with a single processor have a known issue, apparently. Since there's only one storage controller on the unit, it intentionally disables write cache. Oops. I didn't even have to troubleshoot with the engineer. It was pretty much "Do you only have one processor in this?" "yes" "Oh, well, there's your problem right there".
So yea, if you're thinking about an EMC AX4-5, make sure to pony up the extra cash...err..cache..err..whatever.