This article is from the Feline Infectious Peritonitis FAQ, posted to rec.pets.cats newsgroup. Maintained by Erin Miller with numerous contributions by others.
Well, Cornell concluded from the above that vaccine efficacy in a laboratory setting is highly dependent on the challenge. It offers protection at low challenge doses, none at higher doses. The problem is, no one knows what the "real world" dose level is. Another problem is that there are actually two strains of FIPV. Just as there are many different flu strains or cold strains which cause you to get sick several different times with the flu or a cold, because each time you catch a different strain for which you aren't already immune. Type I strain of FIP is believed to be the most prevalent in the "real world" but it is the most difficult to reproduce in a laboratory. Type II is easier to reproduce, but not as prevalent outside. It is not known how effective a vaccine against one type will be against the other type.
So, while some of the studies have found the current vaccine effective against the Type II strain of FIP, there is no evidence either way as to if it will work against the Type I strain.
So that is Cornell's opinion, are there any other points of view? The consensus arrived at the seminar sponsored by the Winn Foundation on FIP/FECV is that the enhanced disease effect is a laboratory phenomenon, especially since that study by Cornell only used seropositive cats in the first place.