First off, thank you so much @theotheo for taking not only the time to test this on your end, but also for the detailed explanation! I really appreciate that. I read somewhere that this forum is one of the nicest around and I can see why.
Also, thank you June for uploading your plugin, very kind of you, I'll give it a try.
For what concerns the test I posted, yes, the example has a quite shallow depth of field, but after all not so extreme.
Thus, I did another test in AE + Frischluft and I can definitely keep pushing it without noticing serious artifacts:
But then I thought that to make this a fair comparison I shouldn't be testing against Frischluft but rather the built-in AE Camera Lens Blur and the results aren't good at all. I always relied on either Frischluft or Sapphire Z-Defocus, therefore, my references have always been pretty much the highest level achievable (at least in the AE territory).
I guess at this point it becomes a matter of whether BMD will ever introduce a more advanced tool or not for this type of situations. If they ever will, it will definitely be a huge gift and will surely be very welcome. But I also realize that would be really an extra gift on top of what Fusion itself is!
Thank again to all the users who responded here.
The more I use Fusion the more I like it. I think it can definitely become an important asset of my arsenal.
P.S. @theotheo I could finally look at your examples on my monitor, in the second round looks like you're getting a very clean result! I wonder though how long it takes to go over that setup and adjust it for different cases. Anyway, I'll study what you did, which I guess it'll keep me busy for a while since I've just started with Fusion