I'm not entirely sure but it seems to me that from the POV of the traveler at close to light speed all the C2's cancel out and the journey appears to operate using Newtonian math? So if you are near light speed and accelerate you travel hardly any faster, but the universe shrinks by the same amount as if you had gone that much faster. You cannot get to e.g. alpha centauri faster than light, but you can get there in a subjective time frame that would subjectively seem like you did.