Well, making the designers of the Enforcer and Steamrunner be of the same race based solely on the pylon-mounted deflector is a bit of a stretch. Especially after there being almost a century of difference between them. If you were to flip it over it'd be a Miranda rollbar, and if you changed the pylon shape it'd look like the lower wings of a Valdore.
You wouldn't need the race to join up with the Federation just so a couple of ships can share a fairly simple and obvious design trait - there are other, more clandestine ways of getting things from your enemy. It's also reasonable that the two had the same idea or the same requirements when they came up with their versions.
It isn't unplausible that there's more than one race with 'rollbar' style design elements. The Orions, Klingons, and Romulans all share bird motifs in their starships. There's also the Defiant and Hideki classes, and I doubt there was any willful sharing of ideas there.
The way I see it, the ISC just wanted better reception for their Frog-Ball games and decided to mount their deflector/sensor suite far from the rest of the ship to cut down on interference. The Feds caught on to this idea a century later when they made the Steamrunner, and that explains why they can get such good broadcasts during the annual Targ races...