Your lights will dim and blinkers may not work at low speeds and when idling. You may be able to compensate by changing everything but the headlight to led's.
Also a minus if you ever want to add a 12v power plug for running a gps or mini air compressor since the voltage would be swinging around with rpm's
On early TWs, where the battery doesn't power the headlamp, you probably won't notice the difference, since the headlamp dims a bit at idle anyway. However, you will have to replace the flasher if you want your turn signals to work at idle on either the old or late models.