Steering effort for manual 16:1 - should it be this high?

The more + Caster, the more effort required .
You are actually lifting one side of the car .
I have seldom gone as far as +5, most customers are very happy with +2, - 3 max. These cars came with 0 to neg 1 from factory with bias tires.
Reduce your caster, there will be significant difference .