Leverage breakdown by PA:
Porcello
H 130
M 360
L 311
Kimbrel
H 136
M 43
L 60
Kimbrel actually has more high leverage PA's.
Another factor is that Kimbrel's ERA is about half of Porcello's, and has been over their careers.
I do agree that Kimbrel would be overpaid at $20M, I'm just not sure by how much.