This flatness argument, especially the DMT vs. Shapton GDLP has been around for some time. My opinion is summation is:
There is absolutely no direct SHAVING advantage from the Shapton stones being lapped to .5 micron versus the DMT lapped stones to .001". None. Skin is pliable and conforms to a keen shaving edge. So....zippo, no advantage on this front.
HOWEVER...............There IS an advantage (the degree of which can be debated) to having each honing stone in the grit progression used being .5 micron flat vs. .001". Less surface variation or greater flatness from stone to stone makes for faster removal of the prior stone's scratch pattern. If I hone a razor that lies flat as it should on both sides of the blade, I've found that it takes less passes on the finer grit stone to remove the prior stone's scratch pattern since I've been using the GDLP rather than the DMT. I attribute this to the flatness factor.
With that said, are we talking about a world of difference in time? Does the DMT's flatness at .001" mean the DMT can't do the job as a lapper? Of course the answer is no on both counts.
There are so many areas of interest in life where those that are deeply interested and even obsessive about an interest will debate the most minute points of the smallest advantage of one thing over another ad nauseam. It's no different with us.
I like my GDLP and think it's great. Not only for it's flatness but also for it's non-continuous surface which reduces stiction. That said, I would never advocate that DMT is crap and everyone should drop a big chunk of change on the GDLP. Both will do what they're supposed to do. The GDLP is there for those of us find an IDEAL is worth spending $280 when we all know or should know that spending a lot less on a DMT is just fine.
Just my 2 cents. One more thing to add; I don't think anyone who buys or is contemplating buying a DMT D8C for lapping should ever feel they'll NEED the GDLP at a later date. There's nothing wrong with DMT.
Chris L