In my theory, always run the smaller gear one grade harder and here is why. In my truck for example I run a 55t spur and 14t pinion, divide that and the smaller pinion works four times harder than the spur. So you naturally would want the smaller stronger to maximize life. Simple hardness scale from plastic > hard plastic > soft aluminium > hard aluminium > soft steel > hardened steel. If gear are near equal in size and teeth then match equal hardness. Anytime you jump more than one level of hardness you risk eating the other up.
I believe HR temper treats their steel, that is some hard stuff that will eat most other softer steels like butter, so naturally you would want to match them all up the same.
If its not broke, then break it for new upgrades.