He's fast enough already to deal with Flashes and I doubt Cain will use the SF as effectively as any one of them. And Prime's cracked mind coupled with the Gems would phuck some shit up, Juggernaut included.
I agree that Prime would be Uber tough, but I'm still drawing a blank on how he get's around the Juggernauts durability. IMO this would just be an endless stalemate.
Of course, Juggy will be able to move fast, there's no denying that, he's not a retard. But BFRing into the Speed Force, Speed steals, time manipulations etc, i.e. Forum Flash powers? Not quite as simple to do. Plus, even though Jay has access to the speed force, he's no Wally.