Maybe it's just me, but if you say you're going to pivotgrab something, it's really a given that you're going to reverse it. And if the situation calls for a boosted pivotgrab, that's a given as well considering you main Diddy. Seems sort of redundant- and only confusing outsiders. But if you all understand it, then it's fine I guess. :/
To comment on your last sentence, yes you do move faster along the ground, but, no, you don't pivotgrab faster. In fact it's slower. Reason being you have to cancel the dash attack in order to boost yourself. That takes a couple frames to do.
If you shield it you take massive shield damage. If the MK goes along the ground I'd rather just pivotgrab it safely and keep my shield big enough for later pokes and other shield pressure not to get through to me. Bananas do what... 4-5% plus a grab? I think I'd trade that small amount of damage for a bigger shield, lol- especially against MK. ._.