Just for grins I went and sized some FC headstamp cases with three different 9mm sizing dies. I sized three cases with each die. They were consistent within the three-case groups. Here's what I got:

Redding Pro Comp: .386 base .370 mouth
Hornady Custom Grade: same as the Redding
RCBS 1980 steel die: .386 base .368 mouth

The seemingly narrow base measurement is because of where I measured it. Since the shell holder blocks the die from going all the way down I measured with the case in the shell holder, along with a thin washer as a spacer to be sure I was measuring a sized portion of the case. I was really trying to quantify the taper and how the two carbide dies compared to the old steel die.

Not a spit's worth of difference between the three to me.