Don't kid yourself, Benjamin is a WR with an OLB/DE frame. There won't be many (if any) DBs that can match his strength and momentum. It wouldn't have mattered if Gilmore was himself last night. He'd still be slammed to the ground by Kelvin 9 out of 10 times.
Not that it matters getting in to what could have or might have... but in most games I think Benjamin gets McKelvin. He is far more athletic and faster then Gilmore. He would have been able to keep up and get in to position much more easily then Gilmore. And, slammed to the ground - nah, because they'd have just pulled the CB back 5 yards and any contact would be illegal. Also, they'd have gone to a cover 4.
From the limited amount I saw of KB I have to believe the best way to cover him in the NFL is to intercept and cut him off in motion with the safety. There is little to not reason to not double team him because there are no viable threats to catch a cold on the team. Double coverage on KB will be done from his first snap in his first game. Against the Bucs you will likely see the CB play inside and under on KB, avoid nearly all contact and let Goldson cover the top. Goldson will likely even be the first player to make contact. Being that he only ran a 4.61 Benjamin isn't blowing by anyone and defenders have a better chance at jumping his route then they do actually stopping him. He's just too big and powerful.
If you can keep him outside it will be even easier but a nearly impossible task to do.