A couple of points
1. SIZE does matter. THAT is the area where the Pats 2ndary is going to show the biggest leap of improvement. With the additions of Dowling and Boddin, the Pats will add 2 CBs to their coverage that will be a lot better adapted to cover the ever increasing number of larger WRs that the Pats will face.
Way too often the Pats gave up catches to big WRs who were essentially covered. Butler's problems came mostly because, despite being in the area, he couldn't make a play on the ball when covering a guy like Braylan Edwards. THAT's were HE needs to improve his game. He can get better at it despite his size. McCourty isn't much taller than Butler, but plays much stronger. Its less a problem of talent, but of technique and match ups.
You want Butler to improve his techniques and instincts for the ball, but also it would help if he didn't have to cover guys 6"'s taller than he is. With the additions of Dowling and Boddin, he won't have to. Thus his productivity is likely to improve as well as the team's overall productivity.
2. Adding Boddin and Dowling gives BB MUCH better match up flexibility week to week.
3. I don't see Samuel as being in the top 15 CBs. You cannot be one of the WORST tacklers in the league and be considered elite. If you were just considering ball hawking/big play CBs. I'd have him up there, but NOT all round guys.
4. Troy P can't hold Ed Reed's jock. He is constantly being jerked around when he plays against the Pats. A very good player, maybe even key to the Pittsburgh system, but he's NOT the best S in the league. That's ED REED.
5. The 2ndary will be good even if Merriweather plays as he has in the past. However if he can take the "next step" in his contract year, then it could be extraordinary. I think we will see continued improvement from Chung's game, and Sanders and Page offer excellent depth. Still it is Merriweather, IMHO, who holds the key to just just how good this 2ndary can ultimately be.