Paul I think the best Analogy for compairing the back preassure associated with exhausts is to think of fuel being added to your car with a tapered funnel.....

The fuel go's in no problems until a point where the flow of fuel begins to backup, meaning that its not as free flowing into the petrol tank. It then starts to splash all over the place covering your hands your jeans etc (you get the picture ).

Now if you were then to take the funnel and chop the bottom off so that the funnel was wider in diameter at the bottom, the speed at which the petrol flows to the petrol tank is increased. Back preassure would be reduced, it would still backup in the funnel to a point however it would allow more fuel to be added than the above more tapered funnel.

The question is not really at what power do I need a 3 inch exhaust but rather at what point and what level/measurement is back preasure detremental to my car.

The above is not meant to be in any way patronising, hopefully there is others reading this thread who may find the above of help.