30%? Well, let's see... area of a 2.5" diameter circle is calculated at Pi x radius squared. That would be 3.141 x 1.25 x 1.25 = 4.9 in sq. For a 3" it is 3.141 x 1.5 x 1.5 = 7.1 in sq. And if you divide 7.1 by 4.9 you get 145% or 45% more area.
From what I have read it is FLOW not area or cross sectional area that you are concerned with. If you have a garden hose that is 3/4 inch I.D. have you ever noticed how it flows about twice as much water as a 1/2 inch I.D. hose? The formula for flow is .25 X Radius Squared (.25xr2) so in our example of the garden hose that is .5 inch in dia. .25x.25x.25=.015625 and for the hose that is .75 inch in dia. .25x.375 x .375=.0351562 That is why the 3/4 inch hose seems to be flowing twice as much water. Granted I am not William Mulholland, but I did learn at a very early age that the hose that is a little bit bigger does flow twice as much water. The mathmatical number that you come up with when measuring the inside diameter of pipe is "a factor" and of course is of value in comparing different size exhaust pipe,however in the real world, you would need to compare the compressed diameter of the exhaust pipe,unless you are using mandrel bent tubing. Hope this helps.
So back to your 2.5 inch vs. 3 inch pipe Question ......using the "flow" formula for 2.5 inch is: .25x 1.25 x 1.25=.390 and for 3 inch pipe .25 x 1.5 x 1.5=.5625 and .5625/.390= 1.44230 SO YES.... the 3 inch will flow about 45% more than the 2.5 inch ...so anywho I guess the way you figured it is as good as the way I did ...unless we are both wrong!