So for my priorities, I concentrated on trying to achieve best power at take off rpm - around 6000 rpm.  The longer length and smaller diameter tubes give the engine lots of oomph in that rpm regime.  Clearly if you wanted an all-out air racer you would more likely choose shorter bigger tubes. 
 
 
Thanks again for the great summation.
 
Ed

Hi Ed,
what is the length and diameter of you tubes?
TIA, Dave McC