Out of boredom, I re-watched GSP's and Hughes' old fights, as well as their championship fights against each other. The one thing that was immediately noticeable was how one-dimensional Matt Hughes was, and how GSP was able to easily dispatched him because GSP was the first well-rounded WW of that era. GSP had excellent scramble, takedown defense, BJJ, and dynamic stand-up striking that kept Hughes constantly guessing the whole time. GSP dictated the fight.
With that said, I do not see GSP and Hughes at UFC 65 (in their primes) being able to beat the current Tyron Woodley.
Woodley has the wrestling, 1-punch KO power, super fast counter striking, excellent takedown / takedown defense, great BJJ, and very high fight IQ. He's pretty much like GSP but with more power and less endurance.
Woodley would KO Hughes within 1 or 2 rounds, and GSP might be able to drag it into the championship rounds, but I don't see him being able to handle Woodley's striking and power.