I use the FastFerment conical fermenter with my Brewjacket and have had a similar experience. It doesn't seem to be able to get more than 20 degrees below ambient.
However, I brew 5-6 gallon batches and a considerable amount of the Brewjacket rod is above the wort level inside the fermenter. I suspect on a fermenter where more of the rod is submerged, it would be able to achieve a larger temperature reduction.
I'm curious, how large a batch are you brewing in your FastFerment? How much headspace is there between the top of the wort and the top of the fermenter, where the rod is effectively not in contact with the liquid?