The main problem with steel(iron) shot, is that it is very light in comparison to the same sizes in lead. That is why there are huge shot sizes in steel, larger than lead sizes, which, except for buck shot sizes, stopped at BB shot.
Now there are T and F and other large sizes in steel shot. Larger, means heavier, but also fewer pellets in a load due to the volume being restricted.
Heavy shot and Bismuth are better in that regard and have shot charges(weight) closer to the normal ducking and turkey load sizes.
Tungsten shot is even heavier than lead, thus even smaller (thus more of them) than normal lead shot can be used & still give the killing power.
I have seen in print that #7 tungsten shot is equal to 5's and 6's in lead which are/were used by many for ducks, myself included. I liked 5's for pass shooting and 6's as well as 7 1/2's for shooting over decs. The fine shot was great for close shooting as it penned the heads and necks, but not the breast meat.
I've not hunted turkey, but in steel, I think a mix of steel 2's and 4's might be good for heads and necks. Finer yet, in Tungten, I think.
Some companies sell "turkey" loads.
The 835 will handle them all.