Doesn't work like that, you don't just simply add the two together and have equal to 5" exhaust. A dual 2.5" setup flows slightly better than a single 3"... a single 3.5" will typically outflow a dual 2.5" setup. Also, the choke points with the factory exhaust are the catted downpipe, the resonator, and the stock mufflers. The exhaust pipe size itself isn't really much of a restriction, not like factory exhaust systems from 10-20 years ago that had large kinks causing restriction to exhaust flow.
Actually a dual 2 1/2 flows almost as much as a 4". Our factory dual pipes are 2.25, and therefore flow the equivalent of about a 3.1" exhaust. Making the choke point in the system the single 2.25" pipe going from the downpipe to the resonator. Downpipe + resonator delete remains the best bang for the buck on the exhaust.
Is there somewhere that you've located this info? I'd be interested in looking at it as well. I've briefly searched for info related to exhaust pipe flow, but I haven't came across anything of real value.
Fluid flows (gas, water, electrons) are restricted (or not) by the surface area of the cross-section of the pipes (pi*r^2), not taking other restrictions, like bends and mufflers, into account.
Ultimate Racing has a single exit full 3" exhaust.
So a 2.5 inch pipe will flow better than a 3 inch? And I know there is a single but I was hoping for a duel 3 inch