I am running MBRP SIDO cat back I like it well but looking for a little more agressive tone.

I don't really want to put in a whole new setup. What are my options and which will produce best sound?

Putting in a different brand muffler.

Or putting in Hi-Flow cats?

 

The catalytic converter change will help, but not as much as if you were to change to a smaller round body muffler. I would look into longtube headers. They increase the volume nearly 30% on average.

 

I wonder how hard it would be to put in smaller muffler

 

If everything is in line and not offset it would be a breeze. Once pipe bending is involved it makes a DIY job more tricky.