Jacob,
While no engineer, I believe the lattitude defines the minimum inclination achievable, but not the maximum. I think reaching orbit is a function of the earth's rotation speed added to the rocket's acceleration, thus it is the final inclination that determines fuel use. It takes the same fuel to launch to 51.6 degrees from that lattitude as from 28 degrees or 0 degrees. (Or maybe less from the lower lattitudes). We originally wanted 28 because we could launch heavier payloads. Russia can't go below 50 from Baikonor. If the Russians foresaw a profit by launching from Kourou I think they would move, what I don't understand is why the Europeans would let them.