Here are three points to consider.

-A reduction in primary pipe Diameter of 1/8 of an inch will move the torque peak down by 650-800 RPM. Conversely an increase in diameter of the header will raise the engine speed at which max torque occurs,by approx the same RPM,for each 1/8 of increase.

-Adding length to the primary pipe will increase low speed and mid-range power, with corresponding reduction in power at max RPM.

-Shorter primaries give an increase in high speed power, at the expense of reduction in the midrange. However,there will be little change in peak torque or the engine speed at which it occurs.


source: straight from Graham A Bell - Four Stroke Performance Tuning.

To take the guess work out of this, you really need to know the timing events of the exhaust valve. What camshaft are you running? Do you know the timing events at 50 thou lift?