the mixture does not burn quicker because of a stronger spark, no. The mixtures speed of combustion is mainly decided by the compression ratio (sort of) the more compressed the mixture, the quicker the flame front.
A weak spark that cannot ignite the fuel mixture on some occasions will certainly loose you power. However, the coil and spark gap determines the sparks energy. The lead will have v.little effect, unless it's insulation has broken down, and it is sparking to the head/block somewhere.