This doesn't answer the OP's question about ProComp AT tires, but there is
a difference between the ProComp MT and the ProComp Extreme MT.
Here is a ProComp MT
And here is a ProComp Extreme MT
I have heard poor reviews of the ProComp MT, and good reviews of the ProComp Extreme MT. Lance03's review of the ProComp MT doesn't sound too rosey either. I have heard that an entirely different company makes these two tires. Cooper makes the Extreme MT, and it supposedly has the exact same carcass, and rubber compound, that Mickey Thompson Baja ATZ and Baja MTZ tires, and Cepek FC II tires have. They're all made by Cooper to the same specs, but have different tread patterns. Mickey Thompson Baja ATZ and MTZ tires, and Cepek FC II tires have good reputations, so it makes sense that the ProComp Extreme MT would also be a good tire.
It turns out that I happen to have a set of ProComp Extreme MT tires on my Jeep. They have 4000 miles on them now and have no appreciable wear. They ride smooth, track straight, and were reasonably easy to balance. Two tires took 9 oz of weights to balance, one tire took 15 oz to balance, and one tire took 16 oz to balance. That is a fair amount of weight, but the tires are smooth as glass, with no bouce at all. On road traction is fine. Off road traction is very very good, ... as good as any tire I've owned (Goodyear MTR w/K, KM2, Cepek FC II). Just last weekend I crawled a very technical rocky ledgy steep hillclimb without slipping a tire. There was 2" of snow on the hill, and this was the first time in 15 years worth of trying that I have ever been able to climb this hill with snow on it. I've tried with dozens of prior vehicles, and struggled in vain, only to have to winch to the top. I was very very surprised that the ProComp Extreme MTs took me over the top with no drama at all. I had gone to the hill that day to be able to test the winch. I had no idea the Jeep would make it over the top. I think these tires are fantastic, and I highly recommend them.
I would think the same thing would apply with the ProComp AT tires. Namely, the ProComp ATs are likely an economy tire, and the ProComp Extreme ATs are a good quality tire that works very well.
Just my .02.