In general the diameter of the header tubes will determine the rpm at which the torque peak from the headers will occur. The length of the tubes will rock the curve around this peak with long tube headers increasing the torque below the peak and short tubes increase the torque above the peak. Since HP is nothing but the mathematical product of toque and rpm (with a constant), yes you could make more peak horsepower with shorties.
BUT, it may be at the expense of torque at lower rpms and the average HP across the rpm quite likely will be much lower. If you plan to drive your car on the street where torque at lower rpms is more of an advantage go with long tube headers.
