No need to excuse, the thing is there is no the 12k or the 16k.
Every stone from every manufacturer works different and can not be easily compared to others.
Both the Naniwa 12k and the Shapton 16k and the Shapton pro 12k/15k are similar in speed: quick.
And they are similar in finishing capabilities: absolutely sufficient.
All of them can and are supposed to be used after a good 8.000 stone.
These high grit finishers are likeable and especially combined with a fine pasted strop
hard to beat in terms of smoothness (as long as your sharpening skills are sufficient!)
In short: what he said.