It sounds like you spoke to someone who didn't know enough to be giving other people advice. Ignoring the fact that stiffness is not measured like that, his numbers are wrong. Perhaps he was quoting the tensile strength of their beams specifically (as that is measured in PSI) or just as likely, he got it the wrong way round - I.e. Carbon fiber's tensile strength (Howe much it can be stretched before snapping) is 4 x greater than steel. As mentioned here:
https://arstechnica.com/science/2015...-carbon-fiber/
Some of the cf I-beams, tubes and channels I see for sale are most certainly too thin and expensive to be a viable alternative to steel. In general, cf doesn't like long flat parts with 90 degree angles so a CNC gantry requires more design thought.
A properly designed carbon fiber part will be stronger and stiffer than the equivalent steel. "Properly designed" is the key point with carbon fiber parts. It makes far more of a difference than it does with metals. A part is not automatically stronger, stiffer or lighter just because it's made of carbon fiber. It is definitely possible to make a weaker part. An aluminum plate can be stronger than steel if it's 5" thick and the steel is 1".
For example of how important cf design is... if you cure a single flat layer of 2x2 twill carbon fiber, you can bend it and tear it with your hands. You cure that same piece of cf in a tube shape, you would struggle to bend it at all. If you stood on the end of the tube, it would probably support your weight.
Where cf has the advantage is that parts can be designed in a way that puts all the strength and stiffness where you need it based on the direction of the forces. As you can see in the video comparing the cf and steel drive shafts. The steel shaft bent and then broke with 1/3 of the force of the cf part. The CF part hardly bent at all before it broke. The cf shaft was designed to take that type of force in that direction.
The better approach with CF is to start with how much strength and stiffness you need and then specifying the part to deliver that. In a diy environment without engineering expertise (which is most of us here), more trial and error would be involved. I.e. If a part is not strong or stiff enough, you add more layers (in the correct orientation) until it is.
Personally, I would never buy ready made cf plates, tubes or beams. Most companies over here charge a fortune for very average parts. You'll pay $400 for $50 worth of carbon fiber. The cheapest 1.5" x 48" round tubes on eBay cost over $80. I can make the same tube myself for $15.
If money is no object and you want to buy ready-made, why not try getting a quote from Comotech? They export directly to America and the rest of Europe. They have specific expertise and experience in designing and building carbon fiber gantry beams for CNC machines. They have the capability to build beams of 12 feet (or even longer). They could make a gantry for you that is stronger and stiffer than a steel one or lighter for the same strength.
I hate the idea of ordering an expensive custom cf part from someone who doesn't understand what is needed in a gantry or how best to achieve it. Starting with cf angles and flat stock doesn't sound like a good design plan at all. It sounds flimsy. You don't want carbon fiber for the sake of it. You have to design the part properly to get the benefits.
If it were me, if faced with the choice of paying a fortune for their suggested approach with cf, or using regular steel or even aluminum beams, I would go with the steel or aluminum.