... but it seems that a CE is as well able to lightbulb as heavy as a FE does.
I would say that that's not true at all early, and closer but still not really true later.
Early: It's not really a GP farm before the NE. That means Lit, plus some build time, plus probably some build time for the GL first (unless you either can't get the GL or you use a GE). Before that time, you can get your first couple of GSs off of libraries. To do that, you've got to look like a SE/FE, or at least like a hybrid with a couple of cities in SE/FE mode.
Later: Once the GL/NE are up and running, a CE can come closer in great person production. The GP farm will be almost as good, but the SE/FE probably has empire-wide advantages: some combination of Philosophical, Pacifism, Parthenon, Caste System. If the CE has a Philo leader as well, and adopts the same civics as the SE, their GP farms will be equal, but the SE will still have its second, third, fourth cities etc. running at 2x or more GPP production, which can contribute even in the face of a 3x GP farm.
Personally, I've become convinced that it's worth running scientists off libraries for the first 2-3 GSs even if I plan to run a CE with a single GP farm later (which I most commonly do). The return on investment for those first few is just too good to pass up.
peace,
lilnev