Hmmmm! Not sure about case hardened plates and whether its a good idea. Here you have a hardened surface(the plate) and a soft surface being iron or aluminum. I've had several moulds with the matching surface scarred/scored by bad sprue plated. Seems to me it's better to use a softer sprue plate and take care of it as you cast.
Warping is primarily casued by lead build up between the plate and mould. I have never seen one so warped that it couldn't be fixed. Normally, this is accomplished by eliminating the cause, heating the plate hot with casting and with the proper hardware stackup for the plate, allow it to cool normally. Tends to go back flat and you get good bullets.
The lead can be removed and by heating, clean with bronze or steel wool. Then, the mould top should be cleaned as well. Proper washer stackup installed. Then the mould top and plate treated (I hear the groans) by pray on graphite or spray moly. Plate lube can be used with good affect or even wax applied very sparsly along the plate edge as you cast. Of course, keeping the heat to a normal level and allowing the mould to cool a bit between casts helps.
So, I'm not thinking case hardened sprue plates is the answer.
There was some guy in the casting community making thick plates for a while. Got several on my moulds and they do better. The problem with them is the screws furnished with the moulds didn't work with the new plates. Had to modify the plates but they worked pretty well.
Shot with a machinist once that used SS plates. Said they worked all right but hard to make. Wasn't around him long enough to follow up on the plates./beagle