I agree in terms of calculating size without any other info, but the comparison is valid if you use a known good configuration as a base. (ie-we know that 2.25" is good for an NA F23 ..... using that base, the size for a J30 at 30% more flow for the same rpm would have a 30% larger cross sectional area which results in a 2.56" pipe or 2.5" nominal) This doesn't account for flow smoothness or laminar-type effects. Also, you really can't control the length of the system, so that variable is fixed.

 

I had a header on my car and loved it (aside from the problems). Sure you lose some torqure but pressing the gas a little farther isn't really all that difficult. After going back to the stock manifold I felt the increased torque but I also felt the resistance to increase speed. With the headers the power was very smooth and easy at high RPMs, with the stock manifold it really seems like you have to mash the pedal down to get the high end speed.