A higher output alternator won't overcharge the battery. The output is controlled by the regulator, this sensors battery voltage and if found to be low it increases the amount of field current to the rotors which then increases output. As soon as battery voltage increases, field current to the rotors is lowered and alternator output decreases.
That's why this unit is called a regulator.
The benefit of a larger alternator is that it can charge quicker and can cope with increased current demands from multiple batteries and the extra drain from high end ancillaries like big sound systems, DVD players for the kids whilst on the move etc
See this link for a more in depth explaination:
http://www.alternatorparts.com/understa ... nators.htm