no, I am not an engineer, but since the smart people havent responded yet I'll throw my understanding out there for you.
It's not a matter of older vs newer, systems are designed to be open/closed based on the heating requirements, hardware (ie boiler design), and regulatory issues (eg if you pressurize it you might need an annual inspection). "Older" boilers that were commonly used in the car wash industry 20 - 30 years ago (and still are btw) often used an open system. And still do. I'm guessing that the newer systems you mention are the instantaneous heaters that a lot of us on this forum have installed over the last decade, and yes pressurizing for those systems works better.
But it is my understanding that pressurizing - in a car wash floor heat system - is done for 2 reasons.
a) overcome head, in other words height. If you have to pump up 10 feet, you need a closed system & a calculated amount of pressure.
b) pump efficiency, for example the fractional amperage taco pumps that a lot of use use work better with pressure.
Pressurizing is not going to get air out of your system, and it is not going to improve the heat transfer in your bays.
Now, it might allow you to better adjust you flow rates which might improve your overall efficiency, but thats beyond my ability to calculate offhand.
