Well, there’s a base comparison for PLA versus PETg, so the CF PETg should be more rigid than regular PLA. Of course a real head-to-head comparison would be better.
I did my LR3 in PLA, but do most of my printing in PETg Foodsafe PETg for that matter, since I end up with a lot of stuff that lives in the refrigerator holding food, or ice… I did not opt for CF reinforced stuff, but went with base PLA.
The largest load, so far as I can see is in the beam for the LR3, with the second largest load being the XZ plates. The XZ plates can be milled from metal, aluminum or steel are available from the V1 store. Much of the actual stability of the beam comes from the strut plates, through the braces. The weakest link there is probably the strut plates, but the braces are certainly important.
If I were concerned about rigidity of a potentially more flexible material, I’d probably just add an extra brace or two to the main beam design, and call it a day.
The XZ plates could be printed with extra infill, but it’s diminishing returns past about 70% or so. Still, I’d probably print them in the CF PETg and if there are any identifiable issues, maybe replace them with milled metal. I can say that I have not identified any problems with the PLA XZ plates that I have, but… I’m replacing them with steel anyway, and now, I know that there wont be a problem.