As for prefinisher, id say that is 8k. IMO a 12k is a finisher. So there is no reason someone NEEDS to go higher than 12k. It will give a keen edge that can shave very well. A lot of us chase the perfect edge or want to find a keener edge than 12k. But when you get to that point the comfort you can find with other stones comes in. Than the chasing starts over.

Just keep in mind that after the 12k the final step is only 2% better in keen. Its comfort levels that start to climb at that point. But the most important stone is the 1k. Naniwa stones for me too up to whatever i choose to take to that last 2%.