If we took prime Royce from the early to mid 90s and put him through a time machine, how would he have done against Matt Hughes?
My thought: He would have done a little better than he did, but not much. In his prime (as now) Royce was extremely loyal and faithful to Gracie Jiu Jitsu. He didn't think anything else was needed. He didnt train in striking, wrestling, he didn't even believe in weight training.
By the time Hughes came around, BJJ was recognized and respected. So guys like Matt Hughes added BJJ to his resume and trained until he was at a high belt (maybe even Blackbelt, I forgot). And that was on top of his exceptional wrestling and okay boxing.
And Gracie, although very good at BJJ, actually wasn't even among the elites of his day like Mario Sperry and Wallid Ismael. So in fact, Matt Hughes was better at his base (wrestling) than Royce was at his (BJJ), plus Hughes had several other tools.
So it's fun to speculate how these fighters would do against each other across time, but to be truly fair to the earlier generations of fighters, they should be compared to fighters in their time and context. And in his time, Royce was very good.