The gilgamesh premades certainly have much better win rates than 25%, you can see them here (as well as my group and some others)

http://na.finalfantasyxiv.com/lodest...kly/?sort=rate

The 25% numbers i calculated were for adders as a whole on average.

Basically you can take those numbers i calculated above as the means for each GC. It doesn't really work this way so don't read too much into this bad math, but to simplify it you can think of having a pre-made as adding roughly 20% to the average win-rate of your GC. If you're an adder with a good premade you're looking at a 45% win rate, storm's a little closer to 50%, and flames is something like 70%. This is pretty consistent with the data I'm seeing if you sort by win %.

Also keep in mind a lot of people don't exclusively roll with pre-mades, especially because the DR roulette is so good, so a lot of the people in the link I posted probably have a higher win rate if you only count pre-made matches. I usually solo queue for 30% of my matches because I gotta get them marks!