Just adding instant grenades will certainly make them more OP.
But instant grenades don't need to come without other nerfs. Throwing range, exploding in inventory when the timer is up, much less accurate, more heat vision to combat the now much better smoke grenades. OXCE has a lot of options there.
Even plain instant grenades at least disable the grenade conga line.
But I already know this dicussion won't go anywhere. :'(