I've heard most people mention that during the daytime, the LEDs aren't all that noticable/bright.
Not sure if that's because of the LEDs themselves or using the stock lens.
If the LED units light up red, the red lens could be problem. If they light up white, it might be better for visability.
Wasn't someone here working on a homemade tail light LED set-up awhile ago?
AKA: Tim & 'The Adventure Cycle' VROC #24567, NEVROC, SteelCity VROC
"When life throws you curves,
Aim for the apex."