I have both the MM Panhard and Torque Arm. I bought the Mac Stainless catback and had no problems using the tails. Mac was smart and mad the over the axle section in two separate parts. Great for this setup. I did have to wrap the passenger side exhaust with wrap from the muffler back. It was bumping the panhard under severe acceleration. The wrap made it completely unoticable. Dumps or side exhaust would be easier though.
As for the two together, major difference in handling. Tightens up the rear more than I could explain. Makes corners a lot more predictable.
As for 500 HP causing issues, don't see it. I used to frequent MM and I call often. If you don't get the HD torque arm, I could see issues. With the HD you should be good for up to 600hp or so.
Just know that a torque arm requires SFCs, and modification to you H or X pipe due to the included crossmember location. Read all the steps in the directions before this install. It looks easy, but there are a few key points that can burn ya.
I have a few good pics on my page in my sig, but none with the exhaust. I have gone on enough for now, good luck.......................