I use P no. 8 material for welder qualification, is he qualified for P no. 1 to P no. 8 production weld as per ASME sec IX.
Info : Filler material for welder qualification & production weld having same F no.
because as per QW 423.1 welder qualifies for P no. 1 through P no. 15F.
Please give your opinion

There are other considerations such as whether there is a change in the welding process, a change in the filler metal product form (affects the WPS, hence it affects the welder qualification), the thickness of the weld deposit, pipe diameter (if applicable), whether he is qualified with or without backing,  position, progression of vertical welds, transfer mode (GMAW), etc.

Then there is the "hook", just because ASME Section IX permits a certain practice, it doesn't men it is a good idea to do so.

Last, but by no means less important, check the applicable ASME construction code to see if there are any modification to the performance qualification requirements.

Best regards - Al