Hmmm I need to update this thread. I put that Flowmaster on and it created some back pressure which caused the Aprilia mufflers to absorb almost ALL of the sound. It really sounded like a Cadillac or some luxury vehicle. It was so quiet and soft which would make a great exhaust for a road trip. What I ended up doing it cutting off the Flowmaster and just adding an extension tube to dump the exhaust further back without muffling it. This created just enough back pressure to allow the race mufflers to absorb some more sound. It's perfect now. I can let off the throttle and drive normal to keep the sound down but when I put my foot in it the decibels crank up for sure. This is pretty crude for now but once the roll cage is built I'll probably change most of the exhaust layout anyways so I didn't need to spend time making it pretty for now.
Here's a video to demonstrate: