Can you kill a turkey with an open choke?....of course. Just get them in really close. Is it going to throw the most effective pattern for head shots on turkeys?..... Absolutely not!
It's probably been stated, but if you want any sort of distance potential out of an open choke your best option is going to be TSS. Behind that your looking at Hevishot or Winchester Longboards. Commercial TSS ammo has become available in the last couple years, but still a pile of guys reloading it, myself included, although I'm relatively new at it. However, if the cost scares you ($7 plus per shell) and you want to stick with lead, you need to look at Winchester Longboard ammo. It is cast in a resin that holds the pattern together longer and in my opinion is often over-choked for close range in an attempt to shoot too far. If you are only interested in 20 yards and in, then any heavyweight load may well be sufficient, pattern the gun to see. Most folks won't stick to that though and are always trying to stretch their limits.
If you choose the TSS route, you can drop-down significantly in shot size because of the dramatic increase in Mass. Lead is 11ish g/cc, Hevishot is 12.7 g/cc, and TSS is 18ish g/cc. Typically guys are shooting #9s and smaller guage shotguns. I've been shooting 8.5s in a 2 3/4" 12 gauge. However, some states have limits (top and bottom) to shot size. These were originally developed based on lead, but are still the law regardless. So if it says no smaller than 7.5, then you can't legally shoot 9s or 8.5s.
Look at ballistic data on TSS and the patterns it throws and you can see why it's devastating to a turkeys head at distances beyond what I believe to be turkey hunting. The best info that you can find on TSS is on the forums at gobblernation.con or the Tungsten Super Shot forum.