5.0L dual in 2.5 single out Y pipe?

[Cobraguy]

Is there any hard data comparing 2.5 to 3.0?

Yes. Contact Flowmaster and see if they will give it to you. Doubt they will though. They will probably tell you that they have the 2.5" in their kit because that's what worked the best. Of course, that's dyno proven too.