The 4*40 is not meant t be fast... propping it for speed is an error. (and a reason for the negative comments you sometimes see about it)

Use the longest, lowest pitch prop you can swing properly with your engine (and not hit the ground durring takeoff) a 11X4 on a .40 is a whole new world in aerobatcs with the 4*40 compared to that 10X6 you think you should use. (and top speed is actually higher because of more PULL with the longer prop) Also... it slows for landings MUCH better with the lower pitch.