Sean,
I thought I would do the same, mufflers on the street and megaphones on the track. The sound is too good with the megaphones and I have ditched the mufflers. It's not a daily driver and the response you will get with the megaphones will probably change your mind too.
During one trip down the highway, I was pulled over by the CHP. My first thought was because of noise. When the police officer came to my door, he asked for a picture of the GT40, as he had never seen one in person. He then asked me to merge back on to the freeway under hard acceleration so he could video the sound on his phone. I thought what the heck and obliged and saw a big smile on his face as I left two black marks in front of him. I backed off at 80 mph and never saw him again, I'll never forget it.
I know you have put a lot of thought in your build, enjoy it!
Mike