At this point I think the MK II and the Bundled Grenade are actually alike in damage effectiveness according to the attribute editor, and the Bundled Grenade is technically superior in combat effectiveness by having no scatter and being thrown faster. The MK II seems far better via the saturation from being on a platform that is built far more, the ability itself is cheaper, and that platform can throw them twice as often at vet 3.
...that being said, I certainly do not think the Bundled Grenade's current performance is deserving of the +15 munitions cost, nor am I certain that the Bundled Grenade isn't just plain bugged or something.
Doesn't the bundle grenade have a random scatter on it? That may be part of the issue.
It looks to me that the MK II (both Paratrooper and Rifle), and Obersoldaten Bundled Grenade has scatter. The Panzergrenadier version, however, should not.