I don't know that I'd pull the gasket off the radiator cap, but cutting the spring so that it can spill into the overflow would make sense.
The whole reason for a pressurized system is to raise the boiling point above operating temperature. Sounds like Evan's has this figured out. I'm not sure if there's a down side to running a pressurized system with Evan's (point it out on their website if you know it,) but I'm not aware of anything that doesn't expand at least a little as it heats up. Okay, water is a special case, highest density is around 3-4 degrees C.
Interesting stuff. It does sort of read like snake oil, but if it does, I can see the benefits. I suspect it'd make the most difference with electric fans, with a mechanical fan, you'll need to get a new clutch to engage at a higher temperature.