One of the basics of rotary combine design is that there are major differences in what is the "best" rotor/concave configuration for, say, 18% moisture corn & 30% moisture corn, or dry shatter prone beans and green stem beans.  Between 80+ bu wheat and 40 bu beans, even down rice and standing rice.

Rotor design is a constant compromise between optimizing rotor performance for one condition but not having it go to heck in another condition.

The aftermarket suppliers don't need to worry about corner conditions and can optimize a rotor for a given local condition.

Just make sure the kit you buy is optimized for your most common condiion and not going to cause a major problem if you get a weird harvest year. jmho.

Jim at Dawn