Well, it seems like you checked alot and also replaced alot of parts. I have 2 things that come to mind. Air in the system and also the boilover protection of the coolant.
First, when the engine warms up and the thermostat is open and radiator full, shut off engine and then see if you can squeeze the top hose closed at all. It should be hard as a rock. If you can squeeze it closed at all, there is air still in the system and it will have to be bled out, usually by jacking the vehicle up under the cap side, with cap off, engine off but warm and squeezing the top hose with both hands real hard and forcing air out the cap.
Second, is checking the boilover protection of the coolant mix, it should be well about 230 degrees and possibly as high as even 250 or more.
One last thing, is the transmission overheating? Do you have an add-on cooler?
If it is not pinging, I would think the octane would be ok. I have a generic answer I made up for overheating,
What percentage of coolant mix are you using? Perhaps you should test the boil-over protection of the coolant to make sure. First of all make sure the coolant level is full. Also, make sure there are no leaves or anything obstructing the airflow through the grille. Make sure you don't have to many fins bent on the radiator; you can straighten them out if needed to. Do you get good flow through the radiator? Run engine until thermostat opens and look down filler neck and make sure you get good streams of coolant flowing out of top few tubes. Make sure you have a fan shroud. Check for air in the system, with the radiator full and engine warm and thermostat open you should not be able to squeeze the top hose closed, should be hard as a rock. With engine off and cool try to squeeze the bottom hose shut with your hand, you should not be able to squeeze it shut or while the engine is running it may suck closed and cause overheating. Of course check for leaks both internally and externally. Check for a sticking thermostat. With engine cool remove cap and watch as the coolant heats up and thermostat begins to open, it should not open then close to open again and spit coolant out the filler neck. Look at the holes in the tubes of the radiator through the filler neck and look for the holes clogged up and if so have the radiator rodded out. Look for good flow once the thermostat opens up. With engine off and engine still warm place you hand on radiator core and feel the entire surface of the core feeling for cool spots. If cool spots are found then the tubes are clogged, have the radiator rodded out. How does the engine run? Any misfiring? Did you recently remove and block off the egr? Is there any smoke or coolant coming out the tailpipe? Any coolant on any of the spark plugs? Any coolant on the engine oil dipstick? Any air bubbles coming up in radiator with cap off and thermostat open?Make sure there is not allot of dirt, mud or even a heavy coat of paint on engine. If equipped with a clutch fan make sure it does not freewheel when the engine is shut off, it should only spin 3-5 times then stop, if more than the clutch is bad. Do you have an overflow tank? Check also that the cap holds pressure.If the auto-transmission has cooler lines that run through the radiator or in front of it and the transmission runs hot then so will the radiator and engine and visa-versa. Make sure the water pump belt is tight and also check the water pump bearing by grabbing 1 fin on the fan blade and push and pull for and aft feeling for any play in the water pump bearing, if so the water pump is going up. Also, make sure there isn’t a lot of coolant running out the small inspection hole on the rear bottom of the water pump, if so the water pump is bad.If you have done any modifications to the engine to create more heat, such as raising the compression ratio then you may want to consider a bigger radiator with another row of tubes. Make sure the exhaust isn’t causing a lot of backpressure due to crushed mufflers or dented pipes. A too lean fuel mixture and/or the ignition timing being too far advanced may also cause excessive heat in the combustion chamber thus overheating. Hot spots on and around the cooling jackets around cylinders can cause overheating. If the coolant has ever been noticed having a rusty color and/or the cooling system never flushed, consider flushing the cooling system.Is the fan close enough to the radiator and is it the original fan and the correct size and number of blades? On some vehicles equipped with AC check that the AC Compressor shuts off when overheating. Also, I believe an overcharged AC system could cause the condenser to overheat and because it sets in front of the radiator than too will the radiator overheat if the condenser overheats.Make sure there aren’t a lot of tubes cut out of the radiator if repaired before.