General relativity *is* a revision of newton's laws of gravitation.
I don't believe a particle's charge reduces as it's speed increases, but rather an electric field propagates at speed c with respect to it's source so as a particle is accelerated to relativistic speeds it is effectively catching up with the field of the particle accelerator and therefore has less force exerted on it, as in the wind tunnel analogy. The electric field of the particle accelerator would appear to get weaker from the particle's perspective but since there is little velocity difference between each particle in the stream, their relative charges would remain the same.
Btw, i'm not convinced that photons are actually massless either, or that c is the speed limit of the universe.