Yeah.

I think, if you have 5/8" thick plaster, the grinder will give you the best and most durable results.

You might, if you feel like tinkering, use white epoxy paint or something to build up the outer lips of of your in-floor return, so that the water flow does not directly impact the plaster. Otherwise, if you still have iron in the water (steel tank sand filter, maybe?) the stains will return.

Ben