I have a volt meter on my Garmin and when using the Oxford grips and the Gerbing up high the voltage drops to 112 and less. If you turn the controller down a little the voltage jumps back to 14.1. So at full heat the accessories will tax the system but at less than full heat the system seems to work fine.
I've got an 04 Wee with 65K miles on the original set up.
I did notice the headlights flickering with the heat pulses of the Gerbing when doing the predawn ride up the mountain on Sunday.
Maybe this is a good time to mention that Eastern Beaver sells stuff to make this less of a problem!