Still, imagining a level 20 Full BAB character with one of these puppies with something like Moar Dakka 10 would be ridiculous. 40 attacks/round. Admittedly, the vast majority are like to fail. But odds alone say you'll get multiple crits in there. Something like Fiery Burst on this thing would be terrifying.
Perhaps you ought to cap how much Moar Dakka can be added? Or have an increasing cost?