I'm still running the old "pro" version of Qimage, would the combination of the fusion extrapolation and the new DFS give visible better prints?
DFS seems something very nice to me.
I do think that they offer visible benefits over Hybrid interpolation and USM alone. Of course it also matters which kind of subject you are printing, but anything with lots of detail should benefit. Whether it justifies upgrading is something only you can decide, but also realise that there are other improvements as well such as a new version of the LCMS color management engine.
I'd say, download the trial version when you have a bit of time, and use the 14 days trial period to run some side by side examples. The trial version installs in a different subdirectory, but uses the common data directory from your current QI version (unless you instruct it otherwise). So you can run your current version parallel to the new one, and look for differences in output quality.
It's a bit hard to predict what exactly the DFS result will be without looking at the editor's preview, because the sharpening seems to be adaptive to edge detail which therefore also depends on the original file you want to print. Being adaptive helps to automatically avoid boosting noise, but also allows to use a large 'radius' setting which has a very different effect than USM, so you may want to also try larger radius settings than you are accustomed to. With DFS the radius determines the number of surrounding pixels that are considered for the sharpening, with USM the radius also changes the weighting of the surrounding pixels for the edge contrast boost (which can cause halo artifacts).
P.S. There is also a YouTube video
by Mike Chaney about the new DFS. He uses too large a radius for my taste, but it does demonstrate the difference with USM nicely.