I've been having a real hard time justifying Grot tanks for anything other than looks. They just dont seem to have a role I couldnt fill better with Killa Kans.
Now the grot mega tank, I think that unit has some potential. A shoota loadout with the More Dakka kustom job should be lobbing out a huge number of shots for the price. Even spicier, however, is the Extra-Kustom Weapon mek job on a mega tank fully decked out with kustom mega blastas. That's an extra shot with every mega blasta for just +10pts. Just ridiculous (and probably an oversight).
Lastly, the speedwaaaagh's extra -1 AP should apply to grot tanks, grot mega tanks, killa kans, and mek guns. So I'd consider figuring out a way you could convert a deffkilla wartrike to be manned by a big boss grot.