Just an observation here: The Gen III steering feel was MUCH lighter than the GenIV, so if we can nail down what they changed between the two generations, this may tell you what you need to further modify to get an even heavier steering feel.......
P.S. I haven't thought this part through yet, but I do know that the front suspension on the Gen VI was made 30% less stiff (supposedly to improve ride quality over less-than-perfect surfaces) than on the Gen III, perhaps this had some effect.
Also, A wider contact patch on the front tires should, effectively make the steering feel heavier, but I don't know what width aftermarket tires would fit....