No doubt Thomas is better than anyone we have back there now.
But football, more than any other sport, is one in which a player's individual success is dependent on his surroundings - scheme, coaches, teammates. We'd be investing draft capital and salary cap space to aquire him. How likely is it that he'd be worth that investment?
We all picture his play in Seattle when we think of Thomas...and yeah, his former position coach is here, and our scheme isn't dissimilar to Seattle's. But here, he won't be playing alongside Chancellor and Sherman, with Seattle's strong and fast front seven in front of him. He's also almost 30, has had some injuries, and the whole reason he might be available is because he's explicitly looking for a big payday.
My guess is that he'd be good. But not all-world, not enough to justify investing a high pick and lots of salary cap space to him. I want to pay Martin, and if Lawrence has another big year, I'd value a proven pass rusher over an aging safety.
Just my two cents...