Yes, two smaller pipes are easier to route usually. also, are you insinuating what i provided isnt correct? lol. though the number i provided are not perfect, they are close enough to rely on.
regardless, the rule of thumb for the past....i dunno...100 years or so for exhaust diameter tubing size is 2.2 cfm per hp to keep the system under 1psi. 475chp x 2.2 = 1045cfm. divide that by 2 because V6 and you get 523 (522.5)cfm. 2.5" exhaust flows right at 500-530cfm depending on how you wanna use ID or OD.. if youre making much over 450whp, a 2.5" exhaust is a restriction at higher rpm. a single 2.75" pipe will flow around 620-650cfm, which is a restriction for a stock red sport but not really enough to matter.
additionally, a 2.5" pipe has an area of 4.49sq in., 3" is 7.10sq in., 3.5" is 9.60sq in, and 5" (because you seem to think 2x as big only means 2x as much potential?) is 19.62(!)sq in. i cheated and used a calc for those lol. i cant do that kind of math in my head. i kinda think were saying the same thing here ultimately, id also much rather run twin 2.5" vs a single 3.5" under most cars, although its really not an issue with a Q50.
anyhow, long story short: 2.5" duals for < 500chp, 3" duals for > 500chp to about , and no Y pipe bullcrap. they ALL neck down to a single 3" or less.
