[quote user="David Harris"] Have you tried using grouped views? I'm pretty sure that the thread grouped view does what you're describing (unless I'm not understanding it properly)[/quote]
Tried this tonight, having received a relevant sequence of messages. Leaving aside the matter that I prefer my folder lists ungrouped anyway, in fact the only additional feature I can find that makes any use of overall message dates within a thread is Group by Thread Activity, but that sorts on the date of the latest message in the thread, not the earliest.
The other difficulty I would have with groups stems from the fact that I like to have my new emails listed oldest first, and I progress downwards though the folder as I read them. So I enable the Sort in Reverse Order toggle. With threads, this does what I want - the threads are in thread order and the messages then in increasing date/time order - but using a grouped view this setting also reverses the order the groups are listed, which I don't really want.