I bought two new 1610s yesterday and upgraded their firmware from B8B9 to B8M9. I checked the reasons for each release since K9 and there doesn't appear to be a reason for either a P9 or S9 release.
M9 and previous releases optimizes media (with a few exceptions) and enabled features lockstep in 1610s and 1620s. P9 enables faster dual-layer writing which doesn't concern us. S9 appears to be a region-specific release and some have reported problems with it.
T9 however promises three improvements two of which apply to us, better media writing being among them so I'm hoping to see BenQ release a version of it for us in the form of a B8T9. I found a firmware site that was offering a test version of a B8T9 (modified from a B7T9 or provided courtesy of BenQ I don't know) but it lacks a flasher and I think I'll wait a while for an official version.