46,
I hesitate to jump back into this but you have a sincere question.
The answer is contained in the posts above.
The primary difference between 333-6 and SA-106 IS NOT chemistry. Gr 6 chemistry is essentially identical to Grade B 106. Its the processing. SA-333 G6 is processed to result in a fine grained microstructure which is suitable for cold temp applications (quenched and tempered). The welding procedure qualification is an entirely different consideration. If you were to take a nominal SA-106 and process it as 333 grade 6 you would essentially have the exact same stuff. There is nothing inherent in the chemistries that make the difference. Even though the specification in Grade 6 shows lower S and P, even 106 is produced every day with far lower S and P than that. In fact steel manufacturers are so good at removing S and P they created the now infamous fusion problems of stainless steels back in the 70's before they started offering higher S materials.
As far as the welding procedure qual is concerned once you run the material through the weld procedure regime the SA-333 Gr 6 is essentially gone, the Q and T in the HAZ is gone. But you do still have an essentially normalized microstructure if you keep the heat input down, which is the emphasis of any good procedure qual for a CVN regime.
And this is why you cannot improve the Gr 6 HAZ by low heat inut procedures (you will still get grain growth no matter what) but you can improve the large grained hot finished SA-106. And since the chemistries are essentially identical the base metal dilution is not going to effect the weld metal AT ALL.
Now, having said this, there is nothing wrong with using 333 G6 for quals. Its just that it doesn't help. And in fact, is detrimental if your qualifying for nuclear where the HAZ has to beat the unaffected base metal. Try doing that with 333 6. It ain't easy.
The boiler code and the pressure piping codes recognize these facts.  And in fact, I learned these lessons the hard way many moons ago, and was enlightened by one of the finest gentlemen in the industry, now the Chaiman of Section IX and a member of the B31 committee as well.