I would go with a 3 inch y into a 3.5 inch single. Thats what i'm going to run with my new setup.

 

Would you mind explaining?

 

I am guessing that the 4" with the supercharger you might be ok. I ran 4" on my silverado and i killed the bottom end. Ended up putting the 3" back on.

 

Bigger is not always better.

 

XLR8N is right about the frictional surface area per inch

3" = 9.42" x 2 = 18.84"
4" = 12.57"

If you multiply these numbers by your total pipe length(s), then you will get the approximate total frictional surface area of your exhaust system. If an equal amount of air exits the engine, the air will flow faster through the 3" pipe. You are also less likely to compromise your low end torque as there is greater friction with the dual 3".

In my opinion, it is better to err on the small side of exhaust pipe rather than the large side.

 

I would go single 4" I think dual 3" will hurt some back pressure is good for low end. With exhaust gases staying hotter they have more area and move faster out of the pipe

 

Im making 3" pipe from each header to the dual 3" in / single 4" out magnaflow catback. Im going to be welding in 2 3" electric cutouts into the 3" piping too. I figure that way I cant loose and it gives me limitless possibilities.
The piping will be about $80 and the magnaflow catback is around $380. Im not sure about the price on the cutouts, but Im thinking about $250 for both. and it will all be stainless steel

 

I'm runnin 2.5 inch dumped infront of the rear end. It sounds pretty good but could be alittle louder.

 

i question the whole loss of low end due to not enough back presure but with a blower i know this isn't a factor. the better it flows the more power he will make.

 

what is wrong with your magnaflows you got now, they are one of the best flowing muffles you can get.