The key to the underslungs is to make them worthwhile in only special circumstances - such as Bug Shot to _maybe_ breach a tanker's armour, or flechette to maybe get more than one warrior when they are too close. They should never be a replacement for the Morita itself.
As for hand thrown grenades, I would be tempted to remove them. A unit throwing grenades is a pain in the bum due to all the lethal zones flying around. In WaW and Modern Combat, we assume grenades are used whenever units start shooting at one another with small arms, and factor this into the rifle stats.
The mid-way point between these is, of course, to give grenades Damage Dice without a Lethal Zone. However, you never want to replace the rifle, so we have found it better to abstract grenades out into the rifle's attacks.
Al that said, grenades will be coming back big tim in the Special Forces rules early next year, but the type of game being portrayed is very different.