I run High Trash 50 CO--Fast ON, with some disc to knock out the smaller iron and some bleed over on the top line. I have used both Manual- low 20's and Auto +3 with similar results in high iron.
Go slow and you will get some high falsing but the good targets will jump through the null and the mid/low tones of the good buttons really sound off. Dig those foil tones too as the small flatties will hit down there.
Get the 6" coil, but I have found that the stock coil with a slow sweep is still pretty darn good in heavy iron.
My most recent site is a continuous null and good targets are jumping out with 2-3 square nails in almost every hole. That with the Stock coil. The 6" did manage a few more targets but not really what I was expecting. So I am confident with the Stock coil.
Good luck
Ron