It is my understanding that the "fine" setting on the SG 250 approximates a 1000 grit. The extra fine diamond wheel is 1200 grit. The diamond wheel should leave finer marks on steel, is this so?
My understanding is that the "Fine" wheel was around 600 grit. 1000 and 1200 are very close together and the jump from the Coarse at around 360 to 1000 would be enormous and wouldn't make sense. Nor for there to be another whole wheel just 20% finer at 1200 grit. I sure could be wrong, however.
In my limited experience, the Fine wheel leaves a grind finish similar to the SB stone. I was using my SG stone graded fine (presumably around 1000 grit and I've been using a 1000 grit diamond plate to grade it), but have not really compare that finish with that from the "Extra Fine" diamond wheel.
Rick