I have an '08 (via serial number) and I can't see any differences except for the RP23 (much different). The website states that there are some "secret" changes. They are so secret I can't tell what they are.
If I had to guess.... and this is a guess. Wall thickness.....= increased strength on the 08.... and they won't tell us because of liability on frames already out there.... for warranty reasons... JUST GUESSING.... I like my 07 and would buy it again.
I have an 08 Epi in Large and compared to my 06 Epi in Medium the seatstays are beefier. This may be just the difference in Med to Large, but the rear end is stiffer than my 06. Also, the left chainstay has a bend in it midway down to allow for crank clearance. Otherwise it looks pretty much like the 06.