No, they don't, focus fire is set to false for any type of stg
Focus fire "true", means exactly what it implies. It provides no benefit.
HMGs, SMGs and ARs have this property set to false, which means they have this "hidden" reroll on a small cone from the model they are targeting.
Conscripts get's a +10% accuracy at vet 3. The 40% acc at vet 2 mirrors that of Grens. Penals are hardly broken at late stages of the game as their main appeal is early on, with 2x 30% accuracy. Maybe the spammility of cons pushes it over the top, but as you say, early to say SVT OP.
Received accuracy through merge remains to be the one of the model at vet 0 and the modifiers of the squad on which it is merging too. Merging conscripts with CE only makes sense to keep a flamer squad in the front.
Royal engineers vet bonuses are really poor compared to fighting infantry squads (no accuracy bonus).
I don't see SVT breaking OTHER factions. I don't see SVT been better than the alternatives other factions have. I can see a possibility of Cons + SVT/PPSH been good.