No difference for 99% seems about right. I've never worked with files from a P65 but I can imagine their size if you have many layers in them. I do feel it's worth it to do processing in 16 bit and if you keep a Master File in all its unflattened hi res 16 bit glory, you'll be positioned well to take advantages of whatever improvements we might see in the future.
A conversion to 8 bit for printing makes sense. My experience with thousands of files from smaller cameras confirms my belief in this workflow, but since my files typically a manageable 400-600 MB at the most, I usually print 16 bit. I have seen the occasional banding/posterization that went away when going back to a 16 bit printing file, and about 1% of the time seems about right. If you don't want even to store huge 16 bit Master Files, once the work has been done, a conversion to 8 bit probably won't make a difference worth agonizing over, even if we do get miracle printers, inks and papers that might have some theoretical sliver of improvement for a 16 bit printing workflow.
Once upon a time, 8 bit images-film scans-were the norm, and more than a few good pictures came out of those days.