The only problem with that feature is performance; the timestamp in posts is not usually indexed (and indexing it will, to some degree, shift the problem onto inserting a new post instead) and as such it can have a large negative effect on performance if you have a large number of posts on your board.
The total posts don't suffer from this as this is already calculated and stored in the user record that XF has already fetched.
I'm not saying a blanket "no", just explaining why it's not something I'm totally comfortable adding at this time.
Fillip