Okay, so I'm finish sharpening two 52100 ultimatums right now and on the 500 as we speak.
I started 120 with slurry, then 320 with no slurry. Here's my observations while taking this conversation into consideration:
The 120 and 220, still without a doubt (IMO)
, are better with slurry raised first. The 320 is where the transition takes place. The 320 Glass Stone (I can't speak for the Pro) seems to be a cross between the regular formulation of the 500, 1000, 2... etc and the 2 coarse stones that are different: 120 and 220. The 120 and 220 have a different color, the abrasive releases differently. The 320 has a bit of the feeling of the 120 and 220, but is the same (or near) color as the 500 and on. This means the 320 works great with or without slurry raised first, and is a very aggressive stone. It just might be my favorite of the sub-1000 grit stones. BUT, a 220 with slurry raised will indeed cut faster, as will 120.
Now, as I went from 320 to 500, yes, I believe there must be a very small formula jump or something here, because these 2 stones behave a bit different.. and slightly similar at the same time. They are both hard and don't dish, they both cut any kind of steel you put them up against, and both are acceptable pre-stones for a 1k jump afterwards. When the 500 gets some swarf it seems to lose cutting power a little faster, whereas the 320 almost doesn't care if there's swarf on the stone, it still keeps ripping steel off your knife. The 500 with a slurry still seems to cut better for me, but not a lot, and it does cause a bit of the "skating" effect as I like to call it. Eamon, I'd love to lend you my 320, but then I'd have to buy another one in the meantime.