I've rebuilt mutt-1.4.1-5 and included this patch, and it greatly improves mutt performance over a slow IMAP connection, like to an exchange server. It hasn't crashed yet on me, and doesn't need to be enabled by default, so I think it would be a valuable addition to the standard build. The patch itself is at: http://dwyn.net/mutt/mutt-1.4-headercache.patch I can submit an actual patch to the specfile if needed.
Any chance this could get into upstream mutt?
Re #1: I don't know. I'll try and harass them about it this week.
At this point, we probably won't add it if it's not upstream. It's being considered on the list for upstream 1.6, though.
hcache support is in mutt-1.5 upstream since 2005-01-26