Remember, for a lot of those 7 channel amps even if you're only going to use 5 channels, you'll still get the benefit of the extra power. It's not quite this simple, but the idea is the same:
7x125=875W total power
Also depends on such things as the output devices, the windings per channel on the transformer(s), etc.
Personally, out of all those, I like the Oultaw 7125 and the ATI 1805. I personally don't like the idea of class G amplification. I...