The comparison is a bit off. If you build a RAIDZ2 wiht a 3x8TB and a 3x8TB pool, you loose 2+2x8TB = 32TB and have only 16TB usuable. 6x2+6x6 is a mixed pool

The comparison is a bit off. If you build a RAIDZ2 wiht a 3x8TB and a 3x8TB pool, you loose 2+2x8TB = 32TB and have only 16TB usuable. 6x2+6x6 is a mixed pool so it's hard to compare to actual disks. An alternative, better, comparison, would be a 6x8TB pool and another 6x8TB pool. This time you loose 32TB to parity and have 8x8=48TB usuable. If you rebuild it to a 12x8TB pool, you loose 16TB to parity and have 80TB usuable.