A 50 manpower discount on grenades saves you... nothing because no one techs grenades anyway. Speaking more seriously, ~80-90% of the time the buff will do literally nothing. In the times where it does make a difference, it'll save you less than 1% of the manpower you already accrued.
The grenade tech would be more important if there weren't so many ways to sidestep it. It only really affects nondoctrinal infantry, so going any sort of doctrinal unit even further discourages it.
Perhaps it'd be worthwhile locking a few more grenades behind the sidetech? The USF cooked grenades and the UKF light gammon bombs (given the Assault Officer) in particular ought to require the tech, else you'd just use those instead.
Otherwise, you might as well scrap the grenade tech entirely and throw its cost on the weapon racks.