Unless you use a LED board, the lights will actually be slightly dimmer. LED's really only work well as direct projection lights. If you notice cars with OEM LED taillights and brake lights, they use an array of LED's that project outwards. Sticking them into a reflector housing doesn't really work that well as the light does not spread like an incandescent bulb.
However, taking a stock 3rd brake light and possibly wiring in an LED board sounds like a fun project. All you would need is 12V.