If you look at the spec sheets for hydronic radiators, you'll notice that the heat output varies depending on the incoming water temperature...free air convection becomes more effective when there is a larger temperature delta. It is more capable of transferring that heat when there is a larger temperature differential. So, I don't think it would matter on the radiator if the pump was going too fast or not except for erosion, noise and power. Blowing air across the radiator will mean that the contact air has a greater Delta T (i.e., will be colder), so yes, you should be able to extract more heat. Again, though, keep in mind some boilers really don't like the return water to be too cold, and that could mess things up.
But, there is a limited surface area in the heat exchanger (i.e., your boiler), and again, similar to the specs for any radiator out there, the longer the run, the more exposure you get and the more BTU's you can transfer (and raise the water temp - back to the hand through a candle example). The run in the boiler is limited, rush that water past the heat exchanger, and you'll possibly cause turbulance, and the media (water) won't be in contact with the conducting surface very long as you pump it past so it won't absorb as much heat per volume, but you're putting more volume past it so you may have the same total amount of heat in the circulating water. A lower water temp to the house radiators means those radiators will be less efficient in transferring their heat. If they were marginal at a designed 180-degree input, on a cold day, they just won't keep the house warm, no matter how fast the pump runs if your boiler can't raise the temp high enough. Running the boiler at a lower temp may mean it is more efficient, and if your radiators are oversized, then you may never notice a difference. Smart, modern boilers use a computer and sensors to adjust the boiler output temperature to compensate for the load, raising it when it is cold outside and it detects the return water temps are dropping more than design. An older boiler usually doesn't have that feature.
Too big is a relative thing. How many problems you see, if any, will be based on the extent of how big is that.