Perhaps the Dev could chime on on what a Difficulty of 8 would do. I would need to review the white paper in detail to know for sure.
If the POW is based on finding prime chains, moving from a 7 chain to a 8 chain is roughly 12 times more difficult. So if you were finding 12 coins a day, as soon as 8 is reached you will on average find 1/12 of that which is 1.
Same thing from 6 to 7 or 8 to 9.
With Primecoin 8-9 in many ways killed mining. It was simply impossible at a difficulty of 9+ to mine and fine primes with that chain length.
Dev thoughts?
I am not the dev, but ive done some research on the matter, and my understanding is that the difficulty between whole numbers seems to act in a logarithmic manner. I may be wrong, but think that as the decimal part of the difficulty increases, so does the size requirement of the starting number of the prime chain, thus making the difficulty increase. Your math regarding the 12 * increase in mining dificulty between whole numbers seems logical to me.
JJ