I've never used the FH, but comparing it to the 40EX Hammerhead is essentially a non start in accessible areas (Hammerhead is on the excavator). The Ground Shark is a beast of a mower, 4" material is essentially nothing. It's rated for 7" material, but that's conservative, based on my experience today. The blade carrier weighs 600lbs, carrying a huge amount of momentum into the cut to knock down larger material.
For knocking down what you described, it would be tough to beat.
"Non start" meaning the 40 EX can't compare to the Ground Shark? or the other way around?
I looked at the company video of the Shark and WOW!
I actually had problems with reference points on your pictures seeing how much you cleared in the 4 minute increment.(feet/distance)