The absolute fastest cutting abrasives are diamond products.
These cost more than typical aluminum oxide based abrasives but they are not exorbitant and the time savings will more than offset the added expense.
For flat surfaces like your cufflink design a simple solution would be make yourself a
lapping plate by gluing a piece of waterproof abrasive paper/film, to a dead flat surface, use a piece of plate glass or a chunk of marble tile for this. Make several, one for each grit you want to use. You then just carefully rub the face of your links against this surface. Add water as a lubricant and your abrasives will work faster and last longer. Personally, I think the jump from 400 to 1500 is a bit to wide for optimum results. Most finishing manual recommend a minimum of 600 grit before beginning a polishing phase. For initial coarse cut down I never jump grits by more than 200 grit. A lot of this depends on how hard your base material is. The SS print material is quite hard in fact.
Industrial suppliers world wide sell a vast array of diamond products in varying degrees of grit from coarse to microfine. These products can take the form of water proof films (like sand paper) also tools with metal substrates onto which diamond grains are sintered. Like hand files, also grinding wheels, and points for use in a dremel rotary tool. Diamond compounds are also available in paste form which can be applied to a lapping plate or small felt polishing wheels.
In the USA a great source for these materials comes from the Gesswein Co. Otherwise just Google "diamond abrasives". I promise that if you try these materials you will never go back to standard abrasives.
-G