My simple thoughts on the energy required to accelerate to the speed of light. I think of it as "cosmic wind resistance". Similar to wind resistance here on Earth. The faster your speed the more "cosmic wind resistance" you encounter hence more energy is required to accelerate you per unit of speed than the last unit. So at from .05c to .1c not much resistance but from .9c to .95c good luck. Just my thoughts.

P.S. Just out of curiosity taking the energy required to accelerate 1Kg from .9c to .95c how much could you accelerate (in terms of c) another (different) 1Kg from rest using the same amount of energy? Please assume objects are in space and not encountering any gravitational or other resistance.

-AJ