I have not used sandpaper to lap but I do/have owned diamond plates including the GDLP.

The only difference between the GDLP and other diamond lapping plates like the DMT D8C 325 grit is the flatness or claimed flatness of the hones. DMTs work well on the Shapton ceramic stones. I have never owned the Norton lapping plate, but the diamond lapping plates need no significant maintenance and stay flat.

You're correct, the Shapton ceramic stones, especially the higher grit stones (4K on up) are very hard. If I only hone a razor or two, I clean the higher grit stones with a soft bristle brush under running water without trouble rather than lap after each session.

The reason lapping services aren't widely offered (Josh Earl was offering to lap stones but I don't think he does anymore since he sold his DMT D8XX 120 grit plate) is that the demand is not very high. The stone owner having to pay shipping both ways in addition to a honing charge would approach half the cost of owning a lapping plate.

Chris L