Hi,

It looks like your custom set-up still kept the single pipe into two pipe design. If that's correct you will not notice any significant difference in just a muffler change, which is essentially waht you would be doing. If you were to add larger diameter pipe or run duals that's a whole different story.
I personally think the Magnaflow is a great product, but with what you have I'd suggest keeping it.

Thanks,
Anthony
TVT Design