The LCpl rank was introduced because of the need for an easily identifiable leader at section 2IC and fire team level.. it's a direct equivalent of what goes on in Army infantry platoons.
Is the measure going to be rolled out across the RAF
No, because the rest of the RAF doesn't operate fire teams in battle, so there's no need for it.
do L/Cpls have disciplinary authority over SAC(T)s?
Yes, because they're NCOs. But in practice an RAF Regiment NCO wouldn't have anything to do with another trade SAC in every day discipline... he's got that rank primarily to to lead and keep his own fire team lads in line. Of course, just as with any NCO rank in the forces, he would have a duty to discipline other trades if needed, but an SACs own line management should really be on top of it's own personnel.
It seems that the introduction of L/Cpl devalues that Techie grade
I don't think it devalues it at all, I think you're probably confusing pay grades with ranks. Not the same thing.