First off, the higher octane fuel will have little effect except will allow you to run the timing where it sounds like he wants to go. That can help some on cooling. The lead comment I won't even address, but I am shaking my head.

Now for a real fix. Get rid of the pusher fan, and since you didn't mention any other form of fan I guess that's the only one. There's a clue in your description. Notice that it ran hotter at lower speed than higher speed. That's because the pusher fan is BLOCKING air flow. The high paid engineers that build these cars originally always try to have the fan on the back of the radiator if they can. Also, using the OEM's for guidance, a shroud around the fan (either mechanical or electric) draws ALL the cooling air through the ENTIRE core. Make sure there are no gaps around the outside of the radiator where it fits to the core support. And if the car originally had a rubber strip across the top of the core support to seal to the hood when it's closed, replace that if it's missing.

BTW, thanks for giving a thorough, well thought out question and information. It makes it a lot easier than the guy who pops up with "My car runs hot, what do I do".