I've done a couple small tanks, one with sifted MGOCPM, and one with unsifted MGOCPM. The tank with the unsifted got a couple twigs and wood chunks on the substrate surface after a couple months. didn't really bother me, and that's about the only difference I noticed.Even if you don't bother doing that, but sift the dirt, I'm pretty sure MGOCPM has lots of bark and such in it, so you'd end up with a lot less... but I've never bothered so I'm not sure exactly how much less.
When I sifted the remaining MGOCPM, I'm guessing I had at least 1/2 a bag left of a 16qt bag.
Ended up with maybe ~1/2 gallon (probably less) of stuff (rocks, bark, chunks of wood) that I couldn't crush or squeeze through the mesh I was using. I imagine it could vary from batch to batch and maybe from location to location.
Also, one of the benefits of a dirted substrate is that the decomposing organics raise the CO2 levels slightly in the tank. I think this effect may be lessened (or lost entirely) if you mineralize the dirt. Not certain though.