Not possible. Great Kanohi directly correspond to level 8 disks, from which they are made. Same with Noble Kanohi and level 7 disks. If the limit for a Great Pehkui is 6 inches, that means it’s the limit for a level 8 Shrink disk, which is the most you can get.
The source for that is the Ministry of Tourism site, and the books take precedence.
I’ve discussed that a bit here.