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Actually wouldn't surprise me, diamonds cut more aggressively than AlO, especially on hard steels.
Sometimes even a finer diamond plate can cut faster than a course, it's like the courser one can't...

Best price I could find was here; ATOMA Diamantschleifsteine
Anyone know any cheaper place in the EU?

Well, that does it, I'm going for a 1200 Atoma.

Yes I've heard that, have to get one myself and try it out.
Any info on how fast they wear when used for lapping?

I'd go for the tomo, a fine plate like 1200 can be damaged fast from lapping, but it's your plate, try it if you like.
... and always use water, water, plenty of running water!