Yup, the 15k to 20k change was nothing to do with any changes, just further testing showed that 15k was conservative.
From what I have seen, an Ultra could be used almost indefinitely on a clean running engine.
If I drove more, I may try one for a 30k run, but that would take me ~4 years at my current driving (plus I have a bunch of other clearance filters to use up first).