Royse is the reason I got into jiu jitsu and MMA. He is the first multi tournament winner but I think Tank would have been a bad match up.
Let's say this happens around UFC 4 and Tank knows what Royce is going to do. Tank and Royce are both fresh. If Royce slips those daisy cutters and gets a clinch I could see him winning in 5 minutes. Tank gasses fast and his sub defense isn't the best.
If Tank manages to avoid the early takedown and employs some sprawl and brawl I can see things being tough for Royce. At this point it might become a war of attrition. Royce will keep diving for takedown and Tank will keep trying to damage Royce. If Tank doesn't KO Royce I can see Royce rallying and winning late.