Not regarding the enhanced version, but... honestly, I don't see what's the big deal with "imaginary" Monkey Island 5 and if ToMI is MI5 or not. Mostly because, there is NO Monkey Island 3 or 4, calling Curse and Escape like that is like calling NFS:Porsche Unleashed NFS5 (though, many people do call it like that... dunno...)
The series' names got themselves into "*something* of Monkey Island", and I wouldn't call that a bad thing. I mean, LucasArts, if it would like, could as well make a NEW Monkey Island game set between Curse and Escape and call it, like, Honeymoon of Monkey Island, therefore transforming MI4 into MI5 (chronologically), and ToMI, into what, MI7? O_O Meh, just messing with your heads
Besides, there is an advantage in NO chronological numbers. I mean, take Devil May Cry series, for example. I do NOT know for sure, but it goes something like "DMC2 is set before DMC1, DMC3 after DMC1 and DMC4 before DMC2". Of course, I don't remember for sure now, but that kinda proves my point that chronological numbers sometimes can be quite useless.