All i know is that the mk 2.5 came after the mk 2 =NB. It is like Bryan said, pretty much the same as the mk2, however there were slight changes to the exterior. Not very easy to spot, but the front nose cone where the grille goes, is a totally different shape to the mk2. The rear end is a little different too. Like i say these are not easy to spot and are very subtle.
As for the mechanically i really couldn't tell you if there were any differences as i don't know.
Heres a little bit about the history of all the Mk's apart from the new mk 3 on my friends site. Hope it clears things up a little.
http://www.japseye.co.uk/MX5.htm