Servos on A123 voltage is no big deal. The current drawn by a servo is dependent on its load not the voltage applied unless its stalled.
The ignition ,however, is a different story. the ignition takes the applied voltage(5V) and multiplies it by 6000 to come up to about 30000v. So increasing the applied voltage to 6.6V means the spark voltage increases to 35994V, an increase of nearly 6000V.
This puts a severe extra strain on all of the components in the ignition circuit as well as the spark plug lead insulation and the spark plug itself.
Not a good idea to leave out that diode.