Adding more yeast than the recommended dosage does no more than helping the colony get established faster. It has no affect on the finished alcohol production, per say, as the production of alcohol is directly correlated to the amount of sugar in the must, which the yeast consume. Two byproducts are a result: 1) alcohol and, 2) C02.
I often add more yeast than is recommended. This is especially true for higher brix (gravity) must. I don't go crazy but add half more than recommended.
What are you fermenting? Or about to ferment?