If Williamson burns up the combine and doesn't show an awful set of hands or a positive drug test, he'll likely go in the first round. Guys like Donte Stallworth were drafted on physical measurables. It's just the way the NFL works.
I can't comment much on Williamson's ability because I haven't seen him play. I am by nature a little leery of guys who didn't produce a lot in college as first rounders. If Dallas could trade down a few spots and land him, I'd feel much better. I was thrilled at the trade down last year.