Not so, the Su-25 is purely subsonic. It is faster than the A-10 in a dead sprint and have its advantages but so too does the A-10. The two aircraft were developed for a similar role/niche but they are very different as well.
You are right, however the indicated performance refer to level flight performance (950 KmPH= Mach0,75), I believe the aircraft reaches speeds beyond mach 1 when engaged in steep dive (dive recovery, watch the video). In any case the airframe will not pose the kind of challenges posed by the A-10 when reaching supersonic speeds.
The problem is not reaching supersonic speeds but rather to have an airframe capable of going supersonic. For instance the A6M2 Zero’s wings would snap into pieces whenever they went supersonic and the lightning’s (P-38) vertical commands (elevons) would freeze taking the aircraft into a fatal dive.