You need every hp you can get though with this heavy pig! If you are going to take the time to get an aftermarket muffler you should always go for the most horsepower. You know, kind of similar to your while your in there kind of guys with pm.
In your defense no butt dyno would notice anything less than 20hp... A real dyno always does though. Magnoflow is ~150 for the biggest sizes available. Any free flow muffler without louvers flows the best. I just disagree in general with chambering a muffler. Part number #14586(2.5 inch), or #14589( 3 inch)meh, it's a tractor motor not a race truck, I really doubt the flow between the two is that different or makes much of a difference.