Just for some perspective, I decided to do some research on rankings of centers throughout the NFL. Here was Athlon Sports' list from this past year (FWIW) ...
Athlon Sports Ranking of NFL's Best Centers (2014) ...
1] Alex Mack -- Cleveland Browns
2] Jason Kelce -- Philadelphia Eagles
3] John Sullivan -- Minnesota Vikings
4] Dominic Raoila -- Detroit Lions
5] Manny Ramirez -- Denver Broncos
6] Evan Dietrich-Smith -- Tampa Bay Buccaneers
7] Chris Myers -- Houston Texans
8] Travis Frederick -- Dallas Cowboys
9] Ryan Kalil -- Carolina Panthers
10] Stefen Wisniewski -- Oakland Raiders
11] Mike Pouncey -- Miami Dolphins
12] Robert Garza -- Chicago Bears
13] Nick Hardwick -- San Diego Chargers
14] Nick Mangold -- New York Jets
15] Jonathan Goodwin -- New Orleans Saints
16] Will Montgomery -- Denver Broncos
17] Brian de la Puente -- Chicago Bears
18] Rodney Hudson -- Kansas City Chiefs
19] Max Unger -- Seattle Seahawks
20] Jeremy Zuttah16- Baltimore Ravens
Source:
http://athlonsports.com/nfl/2014-nfl-player-rankings-offensive-linemen
Matt Miller of NFLDraftScout.com, who also writes for Bleacher Report (still slight blech because it's still Bleacher Report, though BR is getting a bit better with guys like Miller and Jason Cole) also has his own rankings list that pretty much dovetails with that of Athlon Sports. Here is how Miller sees it ...
1] Alex Mack -- Cleveland Browns
2] Jason Kelce -- Philadelphia Eagles
3] Dominic Raoila -- Detroit Lions
4] John Sullivan -- Minnesota Vikings
5] Evan Dietrich-Smith -- Tampa Bay Buccaneers
6] Mike Pouncey -- Miami Dolphins
7] Manny Ramirez -- Denver Broncos
8] Robert Garza -- Chicago Bears
9] Stefen Wisniewski -- Oakland Raiders
10] Chris Myers -- Houston Texans
11] Ryan Kalil -- Carolina Panthers
12] Travis Frederick -- Dallas Cowboys
13] Jonathan Goodwin -- New Orleans Saints
14] Nick Hardwick -- San Diego Chargers
15] Will Montgomery -- Denver Broncos
16] Nick Mangold -- New York Jets
17] Max Unger -- Seattle Seahawks
18] Rodney Hudson -- Kansas City Chiefs
19] Brian de la Puente -- Chicago Bears
20] Eric Wood -- Buffalo Bills
Source:
http://bleacherreport.com/articles/1949460-br-nfl-1000-top-34-centers
However you slice it, Wisniewski for Unger would be a fairly significant upgrade at the center position if the Hawks can get that deal done.