Right, I've tried it with an MPG connected directly to the PP without the smooth-stepper enabled and things are just the same, so it doesn't look like the smooth-stepper is affecting the results.
I even recalibrated the MPG and set the max speed and step / velocity change over to 1000 detents / sec, to ensure that it would be stepping if it was going to, and it didn't, it just ran in velocity mode.
It was quite smooth, probably a fair bit smoother than the serial mpg, but that's probably to be expected.
Not sure where to go from here, perhaps into the house for a cuppa?