The 600 is a great motor and fun machine. I am 260lbs and had a 600 rmk 155 pack me round in the mountains with no problem and was a lot of fun and Surprised most as too its ability. Also had zero issues with it.Dont think current owner has had any problems.
On my 11 6.7 I went from 450 kms to a tank to 1100.... Was well worth it with H&S tuner, Had no luck with Edge. Truck pulled better and shifted Way Better. Never put on higher setting then Tow or whatever was just above stock. Oh and took any lag right out of it.