My friend has the same Remington 870 you talked about, but has the 26" barrel, and using a standared Remington super full turkey choke with Winchester Suprems #5 and is deadly out to nearly 50 yards.

Myself I use a Mossberg 835 and I was using the factory turkey choke with the same ammo as my friend, and I was getting really good results. After the 2008 turkey season I did some research and brought myself a Indian Creek Choke for my 835, I talked to the people there (very nice) and they told me to use the HeviShot 13, 2.25oz, #5, I should get excellent results. I just got two boxes as no one in my area has them, the only place that did have them was Cabelas, within a month I will go and pattern this ammo