Hi Leon.  I am in complete agreement about the need for air filtration.
   On one hand we have the need for filtered air and on the other we
want the performance boost from ram air.  If we supply ram air to a
plenum and filter it before it enters the engine, is there a
significant performance hit?  If so could the ram effect be regained by
using a larger filter, bigger scoop, and so on.  I am building my
intake right now and it will have filtered air with the option of
switching to ram air at altitude.  Do  you think this is a good idea? 
Certainly some dust exists even at 10,000 ft.     Jerry

Jerry,

 I'll be interested in Leon's take as well, but I wanted to kick in here. Yes you can ram air a filtered plenum. If you look at any of the newer motorcycles 600cc + you will find that all the sports and performance bikes use ram air, and the largest plemum box they can fit in the frame practically.  There is a noticeable power increase that I tested on the track. Run the biggest filter you can fit in for the lowest pressure drop. I like the K&N oiled filter element.

Bill Jepson