As most of the post have stated cleaning up the inside of your jet is very important. We have gotten away from this becuase of the large power to weight ratios that we all experience. In comparison to full scale aircraft we are very spoiled by having that excess amount of power. Taking a look back at the ducted fan era, we had to do what ever we could to sqeeze every bit of power out of those units. Ihave flown flown my Euro Fighter with and with out fullbypass and there is a very big diference in performance, but Ido have to say if you have a large engine P160 or bigger you really cant tell. So its all about how much excess thrust is present to actually make a difference.