I'm still testing and will post more shortly.
One distraction: the drawing I used for testing is comprised of multipoints. (An oversight, sorry.) Decomposing to single points produces a slightly different result--the visible edge effects are slightly softer--and the processing is (I think) slightly faster.
I am starting to wonder whether the edge effects may be a side-effect of the massive parallelisation that makes these interpolation methods so fast. (Each overlapping "patch" produced by an independent thread, perhaps.) If so, they may be unavoidable. Which would not be serious, since the edge artefacts are easily suppressed with a light filter on the resulting image. For example, a Blur with radius 3 and power 1, or Gaussian Blur radius 4 power 1, both work well.
I'll post back with a direct comparison between Manifold 9 and Manifold 8, using matching settings (as far as possible), when the Manifold 8 test has finished processing.