So, I don't know about this business of dumping a lot of flash light on the subject, it creates a look that I find rather unnatural looking (and yes, I do realize the irony of that statement, since we are talking about fisheye perspectives of people doing some rather gravity-defying stunts, but I simply prefer the look of shots that use ambient....)
Maybe a better option is simply to go to a smaller format. The mamiya 645 system has two fisheyes- one (the mamiya one) is exceedingly rare and probably ~$1000+ if you do find it, and then there is some non-Mamiya one that is so so, but hey, at least then you get fast shutter speeds and thus can go without flash in decent ambient light, or build up your ambient with some spotlights or whatever. Then there is 35mm... plenty of excellent and inexpensive offerings there and fast synch speeds if you need them, or fast shutter speeds if you want to go without flash.
I have an rb fisheye and I like it a lot for some things, but it never occurred to me even to try fast action with it. If I did do it, I think I'd go to a smaller format, or if MF then I'd go with a strobe, not a bunch of weak flashes. Bear in mind that people did very fast action photography loooong before there were high shutter speeds. There all kinds of inventive things one can do, including having a strobe triggered by the subject passing a certain plane or whatever. I think my nikon sb800 has that capability and command other flashes as well, though I haven't tried it yet, but I was pondering doing some LF hummingbirds and did conclude that it was quite possible.