Jake, I don't have a definate answer for you but I can only assume that the decision to go with 2 pipes instead of 1 is purely for clearence. Where the exhaust splits into two there is a small brace that crosses the car which I assume is for improved chassis torsional strength. It's only a small brace but I can't think of what else it could be for. I'm sure Ford would have minimum clearance measurements to prevent the exhaust knocking or heating sorounding parts so with a 3 inch pipe this may be too close to the drive shaft or floor pan. As for a 2 x 2.25 pipes flowing more that a single 3 inch you also need to take into account the extra pipe wall area in using two pipes over one as the more pipe wall area you have the more resistance to flow there is. So the calculation also needs to take pipe wall area in account. Obviously there's more wall area using 2 pipes than 1. From experince with my FG the mid and rear section of exhuast had no trouble flowing up to 87rwkw over standard rwkw figure. If I was aiming for say approx 400rwkw in an F6 I'd probably use a 4 inch dump tapering to 3.5 inch. If I was going to 400rwkw plus I'd go 4 inch. Benny.