I've killed 3 turkeys with my SX3.  It's got a 28" barrel and a 2" Indian Creek choke, and I've never felt hindered by the length.  I have gone to a 20ga for weight purposes but the length hasn't been an issue at all.  For the choke and load advice, go wtih the .675 Indian Creek instead of a .665 (which is what I shoot).  I wish I had the more open choke because I've seen evidence of some blown patterns with this combo and Mag Blends before.  I would recommend the H13 6's or 7's or possibly even the mag blends if you go with .675 ed.