How good is the inferred multiplier?

You should consider Cyclone III's hardware multipliers as optimal solution in terms of speed and area. They are typically used for inferred multipliers, as longs as you have enough free resources. If you run short of it, Quartus can be expected to select hardware multipliers where needed according to timing requirements. If you don't agree with it's choice, you can change the selection for modules or individual blocks by a "multiplier style" synthesis attribute. See the Quartus software manual for details.