Hey Stranger! Good to hear from you.

I don't have a copy of D1.6 on hand to quote from, but remember peculiarities in this and D1.1 about this issue.

First we have to fully separate performance from procedure quals. The ultimate goal of a performance qual is for the Welder to demonstrate the equipment operation skills necessary to produce a compliant test plate. The test plate is only an example of what is to be seen in production.
Now this being said, CS and SS have very similar actual welding properties. Basically, if you can weld on one, you can weld on the other, often with the same or unmatching filler metal, and still produce welds compliant to non-destructive requirements. (I know, I know, not in every case, but just in general, especially with austenitics)
The end result of a poorly matched material and filler combination can range from cracking to accelrated corrision, etc.
However, in the case of a WPQR test plate, that doesn't matter as it's only scrap, especially when the option to accept through RT rather than bends is used. What matters is the Welders performance.
Now I figure that years ago, some guy or guys on the AWS committees had a problem with spending gobs of money interviewing/testing welders on high dollar material. They were then able to achieve cost reduction through allowing testing to be performed on similar rather than exact materials.

But... Pqr's and WPS's are a whole different story. These have to prove out the fatigue and corrosion resistant properties of the end product.

Tim