I'm not sure without testing if that pipe will work ok or not. Typically they do need some fine tuning even with the stock diameter.
The quick and dirty answer is that the tubes have different aerodynamics basically. The air can hit the sensor from different directions and not be straight, there can also be turbulence at different air speeds. This is why some MAF tubes use an air straightener screen or mesh. There are other reasons as well but thats the idea.
If you change the MAF tube its always good practice to check your fuel trims and use a wide band to make sure your open loop fueling is still on target.
In terms of air temps the subaru MAF sensor has an air temp sensor as well that uses calculations in the tune to compensate for those temp changes.
Originally Posted by dvrmstrng
Question: If the K&N Typhoon Intake uses the same diameter pipe and location for the MAF sensor why would the basic OTS Stage II Map not work the same as the stock box, aside from the fact it may be sucking warmer air.