As far as the angle the pipes come together, I haven't seen any aftermarket pipe that does it much better. I think that is a moot point.
As far as a question of my math. No I include everything in my calculation. Gotta remember, I am a serious math nerd. I do math and physics for fun in my free time.
Marginally to big is still to big conversely, I am not splitting hairs, and I have no particular interest in telling you that you are wrong. Believe me, I am not just telling you that I believe you are wrong just because I want to, I dont really have any particular grudge against you. Depending on the amount something is oversized, the amount of turbulence in the border area can slow exhaust velocities even more than a slight restriction does. I take into account the effects of heat, friction, turbulence, scavenging, and other flow dynamics when I state my opinion that a well designed 2.25" system would perform the same or marginally better than a similar 2.5" system. Especially when used in conjunction with appropriate tuned headers. Though I believe that a true dual 1.75" pair would be the best solution.
Either way, I still plan on proving my calculations at some point. If I dont ever get around to finding a T-Green SVT that I can buy at a time that I can afford it, I will build a system to fit a standard V6 and work with someone who has an unmolested SVT to work out the exhaust question.
Other people are apparently going to take up the y-pipe debate. Only problem I can see with testing the Y-pipe would be inequalities in exhaust systems. That is to say, a person would need 2 exhaust systems to properly conduct the testing, one would have to be 2.25" and the other 2.5", and they would have to be equal in design aspects, the only difference between the two would have to be in the absolute volume they can flow given their sizes.
Now of course, understand that I am saying all this based upon the best power combination I can imagine for a naturally aspirated duratec under normal spirited driving conditions.
Though, we are veering off topic.... probably should stop here, move to PMs or start a new thread.