I suggest you to compare the yield strength values in ASME II D and in Special Metals' brochure for the elevated temperatures.

If they are the same you may need to check the strain value to consider. At limit temperatures the strain is almost uncontrollable. Check it with client and manufacturer for the purpose.

If the compared yield strength values are not the same and ASME considers the lower values for the same elevated temperature, I would not recommend to go further, you either change the material (if you can find better) or try to reduce the metal temperature that ASME accepts for the stress and the strain.

When you work in the creep range of the material and especially on the higher limit you need to take the strain into consideration. I don't have the code with me to direct you on the issue. But I think you understand correctly what I do talk about.