As much of a BJJ fanatic I am, I am dubious that Rickson would be able, even during his heyday dominate today's top MMA fighters. His striking is suspect, his wrestling isn't on par with top tier fighters, but, when the fight gets to the ground, he would do EXTREMELY well. Because he normally got top position in his fights, we don't know how good his guard would have been in Vale Tudo/NHB/MMA.
Ricardo Liborio of ATT said Rickson made him feel like a white belt, so did Xande and Saulo Ribeiro, so you know his jits is top notch. I just don't think the rest of his game is there. Funaki put a helluva a mouse on Rickson's eye, and he isn't known as a top tier striker with good TD defense. I wish he would have fought Ze Saku when Saku was running through Royce, Royler, Ryan, Renzo.