re: Shaping Small parts using AIR hold down.

Actually I was surprised how well the grinding worked when I changed the size of the rubber seal.  It did take off once, but that was when I press hard on that grinder. When you see the depth of the grind marks you may appreciate the forces. 
Normally I would be working on thin stuff ( < 3.2mm) which is a tad harder to hold otherwise. Tad hard for porous wood, but can be fixed (sealed) with a layer of masking tape.
I won't say it's vise jaws pressure and I probably wouldn't hold it with the air when "carving" that thick material... don't know, but I was trying to get a feel to how much holding power it had under a bit of abuse.  
Vises are great for straight edges... circles are another challenge. 
I suppose my dodgy left arm makes holdings more of a challenge than for most and that's why I'm always on the lookout for better alternatives.