I know PFF had some nice things to say about Sherman last season, but in the first half of the year he was a legitimately below average corner, and in the second half of the year (against mostly bad QBs) he was good, but not close to his 2012-2013 peak. Overall I would say he was merely a good corner last year. Though in fairness to him, he was likely hindered by a new DC and a banged up supporting cast. Still, there was a fire Sherman used to have in his early years that got much dimmer after he got paid.
The rest of the corners are #3s and #4s to me. IMO Lane isn't quite as good as his rep on these boards, and he's hurt all the time too. Lane is basically the anti-Maxwell, fewer boneheaded mistakes but he creates very few turnovers, doesn't dominate with press and has struggled with durability. If we are talking about outside corner, I'd much much rather have 2013/2014 Maxwell over Lane. Lane is a good player but there are sound reasons for why he got $6 million and not $16 million. Shead is sometimes solid, sometimes replacement level. Burley seemed like he was coming on last year and then he just disappeared, that said I do like Burley as a #4 corner. Simon is like C-Mike in his first stint, talented yet still had a lot of work to be done (and time is running out).
Browner is probably done playing corner in the truest sense.
Tye Smith I like quite a bit, but only in very small sample sizes. I think he'll be good, but it's way too early to say for sure.
Talent-wise I think Seattle is well above average at CB but they are currently only a shadow of the 2013 unit. If Sherman returns to his dominating form that will immediately change things of course.
Talent isn't all that matters though. Kris Richard suprisingly struggled with the secondary in his first year as DC. He didn't even seem to trust his own players and called a lot of soft zones. If the talent steps up, and Richard trusts that talent more, then we could see a nice boost to the secondary. But I'm kinda doubtful that happens. I still think Seattle is going to win a LOT of games this year, maybe even the most wins ever, but it won't be the same way they won in 2013 with suffocating secondary play.