The cfbot thinks this patch no longer applies, but it works for me, so still set to RFC. It looks like it's because the thread to remove the v2 FE/BE protocol was still attached to the commitfest entry. I've deleted that, so let's see if that helps.
To answer the side question of whether it makes any difference in performance, I used the blackhole AM [1] to isolate the copy code path as much as possible. Forcing lookahead seems to not make a noticeable difference (min of 5 runs):