Although making lightning strike an instant cast woud be a nerf to the hybrid specs, it really wouldn't be very different besides maybe hurting pve damage some. Since the global cooldown is 1.5 seeconds and the base cast time of Lightning strike is 1.5 seconds it would be about the same and would certainly give us more mobility.
-Polarity shift makes us immune to pulls and knockbacks for the duration
-Force slow makes our dots unable to be removed
I don't mind not having LS instant- you could still make it cast as long as you could move while casting, kind of like how steady shot works for hunters- a low damage (and LS already is very low damage) ability you can use while moving to build resource (which is what LS does for lightning spec).
About the cast time of it being instant- as you said, it's the same as the GCD. I think alacrity should effect the GCD though- alacrity as it stands is the most useless stat in the game for everyone, first off it doesn't give much per point- second, unless I'm missing something it doesn't effect GCD.
If alacrity were to, say- decrease spell casts, GCD and increase resource regen it might actually become a useful stat for once.