[fritsch]

Now that there are some substantial performance improvements in the pipeline (many, many thanks to all of you involved in this voluntary task), is there a possibility to implement some more sharpening but apparently more CPU hungry upscaling options such as Spline36 or Spline64?

If you do it, there is a possibility. If you don't do it ... it's very unlikely. It's not on my list.

Edit: I really hope you would not do this on the CPU at all ...