If you can adjust the carb (via adjustments, needle/clips/ jets) to compensate for the extra air then there is no such thing as too much. At some point you will get to the limit off the carb, but even if you removed the airbox and filter system you should be able to adjust the stock carb to compenate for the air it can ingest given it's throat size. Of course an ATV in the real world has to have the filter enclosed and a measure of water protection given the intended usage.I thought this too but when I had the idea to make a 4 inch inlet for my snorkle everyone told me it would get too much air.
That is a Myth. Depending on the valve design (overlap etc.) you can actually get less horsepower from a larger exhaust. An engine needs a certain level of backpressure to keep the charge (air&Fuel) in the the combustion chamber. If you have too much overlap and too free flowing of an exhaust the air/fuel charge will be partially "sucked" thru and not used 100%if you allow more air and more fuel, u will need the burn-off (exhaust) pipe to be bigger too to allow it to escape.