They PROBABLY already are 6:2:1 from BMW but regardless, sure, keep the factory stuff up front and play with the headers back to get the sound you want.
Just FYI, straight pipes are LOUD, the cats will help some but my experience living in NYS was straight pipes=noise tickets, not sure how NJ is....I live out in the country in PA but even here they write noise tickets. In the video of my car running you are hearing these:
3" borla XR1 race mufflers...yes even most race tracks have noise rules these days. something like this drops the noise from say 115-120db to 100-105 db, still loud but not painfully loud so ticket still possible but less likely.
I also have 2 quieter 2" mufflers with valves on the borlas that I will will be normally closed so normally quietish, then have the ECU on based on air mass flow so over 200-250ish hp (stock 308QV btw) it will go from quietish to loud so as long as I stay mostly off the throttle in town, I'm hoping to avoid noise tickets. Even the borlas aren't too bad at lower power though so maybe I could save the weight/complexity but I was scared I needed a bit more flow and added 2 more mufflers to get it...and then more parts because why not right?