Small channel sizes (5 MHz & 10 MHz), and large channels (40 MHz Turbo) were all features specific to Atheros radio cards. Mikrotik implemented support for these features as users demanded it.
However, Atheros has not produced a new radio chip that features the small channel ability and 802.11n features, until they do this there is nothing Mikrotik can do.
I have emailed Atheros, and Ubiquiti on the matter to find out if Atheros is even going to continue pursuing small channel. I have yet to hear back but considering that this feature is pretty much solely used in special cases (WISPS, PtP equip, etc.), my feeling is they will be abandoning it.
802.11n should have incorporated the ability to use small channels in its design, IMHO this is a huge design flaw that will rear its ugly effects as more people start using wireless in high population density areas.