MIA is the worst major airport US POE IME; and I doubt more CBP employees at MIA will do much to change the experience other than perhaps reduce the time trapped there waiting for admission -- but even that would be a material and general improvement unless the newbies' attitudes are substantially worse than the attitude of the current lot there.
APC and GE kiosks have already helped, but more still needs to be done for peak hour arrivals and visitors from Europe or South America.