I've got big ears to match my big hands and feet, so none of the "Low Profile" models work for me.
Any air leaks or marginal contact of the earmuff will allow the more powerful blasts through.
Weak batteries will also lengthen the attack time on sharper sounds. YMMV, but I find NiCad and Photo grade batteries work best - something to do with the high current pulse needed to squelch the amplifiers and the run of the mill batteries not being able to produce a current spike with a steep enough rise on the leading edge.
Also, do the same guns being fired make you wince when you are wearing non-electronic hearing protection with the same NRR?
FWIW, I use the full sized Caldwells, don't remember the model number or NRR, but they were fairly inexpensive. Anyway, they effectively shut out a Barrett .50 Cal as long as I stay in the 90 degree quadrant directly behind the person shooting it.
I don't own one of those monsters, nor do I have any desire to shoot one, but it sure is fun to watch others do it.
Hope some of the thoughts above help,