After being let down that I didn't think it was coming today, we had a knock at the door just before dinner time... My N64 arrived back from RGB Rob, with a fresh new RGB board! We abandoned Donkey Kong and hooked it up to the OSSC. Stoked with the results!
I've heard folks say that the UltraHDMI mod results in a superior picture due to undoing both sets of filtering that the N64 does (it's my understanding Tim's board only undoes the second), but I honestly don't mind. I think the N64RGB comes out as aesthetically pleasing as the PS1, which I think is pretty high praise considering this was never my favourite console in terms of graphical output.
So Duncan and I played Mario 64 again after dinner and put another couple of stars on our save, and I even gave Wave Race a go and I don't absolutely hate it... it was a blurry mess before and very hard on the eyes, so I am beyond thrilled.