It would be wildly affecting physics. The units no longer line up. E=mc^2 is the total energy of a system, based on its mass, or the total mass of a system, based on the total energy. If you multiply in another 300 000 km/s, then the extra m/s just do not make sense to get the energy units. The unit of energy is Joules defined as J=Nm=kg m^2/s^2. Notice that the units work out to mass times a speed. To talk about the total energy of a system, then the speed must be the speed of light, often represented as c (at least in this formula).