A 12Ga, with a 26" barrel, in modified choke is a great all around hunting gun. It would do fairly well in clays also, just don't try shooting doubles.
As far as Turkeys go, I would probably recommend a tighter choke (Full).
IMO, it is not worth it to have the barrel threaded for chokes. Think about it, another barrel in the choke you want in the classifieds would cost you $55 and up (depending on type barrel). If you decide to send it in to get it factory fitted, it would cost you $55 per regular barrel plus S/H + fitting costs. If you wanted a screw in choke barrel from the factory, it would cost you $85 + $27 (per choke) plus S/H + fitting costs.
What would it cost you to have a gunsmith thread the gun for chokes for you, and then you have to look for the chokes that the gunsmith threaded them for.