Well, just to add to this...
I upgraded to 10.9 last night. There was a lot of md_stores/mdworker activity that happened for a while (as expected) and then I opened Mail.app...
Several (4 of 8?) CPU's suddenly were pegged. Part of it was internet accounts and part was Mail.
It took a while for Mail to even respond (an hour?), but the CPUs were still pegged. After 5 or so hours I gave up and put the system to sleep.
Suffice to say, this morning, it ran out of application RAM. (Of the 8GB of RAM, it was well over 5GB and Mail.app was the only app running other than the Finder.) Every time I would launch Mail.app it would run out of Application RAM within 2 minutes (prior to being responsive - meaning I couldn't disable accounts, and yes, I tried disabling networking, but this was all just dealing with everything on the hard drive.)
I eventually gave up and deleted my ~/Library/Mail folder and started from scratch. It's not going to be fun downloading all of my mail again (I know I was just shy of 7000 *unread* messages from 7 different accounts - 4 of which are gmail, and the largest of which has ≈130k messages in it) but if it works, I suppose it will end up being OK.
At least I didn't have any POP accounts.
Thus far this has been my only (yet quite major) issue with 10.9.