I did not find 250 OP. Grenade launcher is a skill weapon and it's quite difficult to use effectively in combat. However, experienced players can wield it quite well and land many devastating blows.
Nevertheless, I believe the grenade launcher wasn't OP and didn't need the ammo count to be nerfed. I don't think it is a big deal though. It did pretty much reward whoever bought it to be able to stock up 250 regular grenades in combat which did offer an advantage against people who didn't buy it. However the advantage really didn't effect immediate combat.