Interesting stuff here fellas. Having raced with Evans for many years now (sometimes illegally) I can shed a tremendous amount of light on the situation.
The vapor doc is absolute gold. What I was experiencing was exactly that in our motors. These hot spots in the heads with traditional coolant (and plain water) will put a lot of air into the system. I tried massive spring caps, swirl pots, and darn near everything else you can possibly imagine to tame the beast going north of 600 real horse. I always have folks that NEVER over heat come to the track and by lap 3 they've boiled over, quite comically as an I told you so. Doing a single drag pull is absolutely nothing compared to track use. Do that drag pull, hard brake to 30 or 40 mph, then do it again. Now do that for 5 mins, or an hour. Most just don't understand until you've been there, but I digress. It's always that the ducting is wrong, the tune is off, etc.. While do have an effect, the typical passenger car is hardly designed for this amount of thermal abuse. Add in more power, a front mount, removed or hacked ducting and you have a recipe of disaster. I spent years getting to where I'm at now and still are progressing and learning what to takes to keep a higher HP car on the track. I'm not going into a soap box session of all the wrong things people do, and from here forward let's assume it's all correct and I'll my car as my example as I have the most experience with it obviously.
I still run a 22 psi cap on my system. Why? It was already there and though the Haltech telem, I monitor coolant pressure. There is a direct correlation between temp and pressure in a pressurized system, mainly from the expansion rate of the fluid. (tossing out the boiling point of water and coolant pressure in this part as we are well below boiling of Evans) What the Evans coolant won't do is vaporize in those hot spots. That is pure gold in any cooling system. This keeps the top of the motor from collecting air gasses, which in turn allows those hot spots to have a liquid present to transfer coolant to. In a water based system, they flash boil and the hot spots get worse. You won't see this on the gauge, but it's happening. When it does show up on the gauge it's too late. With Evans, it's cooling more efficiently by keeping fluid on all of the surfaces all the time, another gold merit there.
I would absolutely run an expansion tank, it DOES EXPAND. Some call it an overflow. Sure, if things go wrong its an overflow, but that plastic bottle (mine is aluminum now) is for expansion. It gets hot, the level goes up. It cools down and it's drawn back into the motor. That way you should never have any air under the cap, period, ever, never ever.
Let's talk temps. You're on the track, watching your mirrors at 150mph, this guy in 'vette is right on your door and the Porsche is diving inside. You're downshifting, coming off of full brakes, turning in and suddenly you have a temp light. This is scary. Are you shedding water now? Boiling over and now have no coolant? In a water based system you need to get off the track.(most folks target 220-240 F for this) There's a very good chance you are boiling over and are most certainly having vapor flash hot spots in the heads. With the Evans, just roll out of the throttle, or better yet, have the ecu drop boost, tweak timing and add a wee bit more fuel. It's consistent, manageable, and predictable. They say you will see higher temps with Evans. I have not. I run a stock thermostat too. In fact, up until 3 weeks ago, I was running an Autopwr Ebay radiator with a couple of crappy fans on the back WITH a front mount IC. What I do see is what I described above, the climbing of temps as thermal saturation is setting in to EVERYTHING. This happens on every car and the only cure is a more efficient cooling system in terms of radiator, ducting, or air flow. (maybe all 3!)
If you want to do an unsettling experiment, put some in a pan with a candy thermometer in it. Looking at completely motionless liquid at 300 degrees when you know it should be boiling is kinda freaky. It's great stuff and it works as advertised 100%. I probably wouldn't drink it though, it smells coolanty.