I think Silva is a little bit better then Sak, but as said before by ACE, Silva is Purple Belt in Jiu-Jitsu,and as he says, hes not as good on ground as many other jiu-jitsu praticioners. As a striker noone matchs him, and since he knows some of Jiu, its very difficult to keep him down, there you get a perfect combination, good striker and no pin on him.
About Belfort and Silva, i think Silva had a lack of experience on that fight. Belfort is a good boxer, but dont work with his legs very well, i think its better for him take his fight to ground, since hes a great Jiu-Jitsu fighter. As a Striker Silva is light-years ahead of Belfort.
And Royce Gracie is not TOP 50 BJJ fighter, but hes still very technique and has a lot of experience on MMA. Its still a tough guy to fight.