The aluminum content for creep strength enhanced ferritic steel, like Grade 91, is limited by specification (SA 335 or other) to a maximum content of 0.040% by mass. If it more than specified it will alter how nitrogen, which is another controlled alloy element, will form the necessary vanadium carbonitrides to provide creep strength. Aluminum and nitrogen balance are carefully controlled in CSEF steels.
Here is some good background reading on CSEF steels if you are interested. I know both of these authors personally because I have a consulting agreement with EPRI.
