I guess one explanation lies in how many peers they have, but the NATed client is never even able to make the announce to the network even though it gets block updates just fine. If that's the general state for NAT clients then that needs to be rethought. Else I'm leaning towards an OS X specific problem in .24 that might or might not be related to the weird disk writes.
I think it might be an OS specific issue. My Win7 client runs behind the firewall only ever has 8 connections but so far all my transactions, with or without fees have gotten their first confirmations in less than 1 hour.