Kaman,
Not likely, the Su-25's straight wings make it nearly impossible for it to ever reach supersonic speeds regardless of level flight or in a dive. Very few straight-wing supersonic aircraft have ever been built (the X-1 family of rocket planes being the only examples I could find). It has less to do with the strength of the wing, and more to do the SHAPE of the wing. A wing design strong enough to withstand the stresses of transonic flight is fine but in this case it causes so much drag that supersonic flight would be nearly if not impossible. Further, if it was capable of such things, there would evidence of the Russians testing this idea and there is none and there is no limit to information on the Su-25's development nowadays, in fact I have a rather detailed book that discusses that very topic..
Also, the A-10 was never built with high speed in mind in the first place. It was designed to achieve a speed perfect for engaging stationary and moving targets on the ground and in such cases, the slower the better. So of course it's airframe doesn't allow for supersonic speed, it was designed that way from the start. The same is true of the Su-25. Such speed isn't desirable for such aircraft in the first place.
