Henderson still cuts weight, just not as severely. GSP, I don't think is THAT much bigger than Benson. Just Thatch is huge for 170. I think he could pretty easily fight at MW.
Thing is, at 5'9", Henderson is shorter than many LW and even FW fighters, so even at LW, he generally does not have a height or reach advantage and has to use power, skill and stamina. But yes at WW he could be a fresher fighter with somewhat of a speed advantage. Hell, Hendricks isn't that much taller than Benson, but he does carry around a fair amount more weight.
I think there are a lot of guys at WW who'd give Henderson trouble, but I think he could do well there.
Hell, if this last fight was a three rounder, I still think he won rounds 1 and 3. Might not be a bad career move.