Indeed our feedback from customers over many years is that changing the header will result in preference for the SuperQ. That's why the RoadsterSport II product description, and all the header and combo product descriptions, direct you to the SuperQ if considering a header change. And once in the SuperQ product description there is a long discussion of the reasons but in brief that's the choice we designed from the ground up to control everything that happens when you install a less restrictive header.
Nonetheless, sound preference is very subjective and there are many customers using the RSII with header (and all our other mufflers too). We have younger customers using our Race muffler with headers, and our Street Single and every other choice we offer. But again our experience year after year is that most customers end up wanting the SuperQ with header change and going with less muffler tends to get a result that is too loud, or too much rasp, or too much drone, or some combo thereof, etc. Best header for 2.5 for best power is our big 1.8 inch, which explained in more detail in the product description for that item, but obviously bigger pipe will also send the most sound down the pipes to that RSII muffler.
Cams really do not make power, they shift power. The stock cams for 2.5 were designed to get a heavier car off the line and moving (as installed in the Mazda3, Mazda6, CX5, etc). Thus, the focus of the stock cams in the 2.5 is not peak power but low and midrange torque. What you are doing with adding cams to 2.5 is changing the shape of that power curve, moving the peak power further up the rpms. You get a higher peak power, but you really have not added power, you have moved the peaks from low and mid range to top end. Thus, indeed if the question is how to make more power, the answer is Header for that 2.5 install because stock header was pathetic for even the 2.0 motor.
This does not mean you won't want cams with your 2.5, I really like Mike's 1.5 cams with any 2.5 install (see Mike at Xero Limit for those), because it gives a power curve that is more what most want in a sports car. The counter argument is that there is nothing wrong with making an NC feel like an old school torque hot rod using stock 2.5 cams that focus on low and mid range torque and we have plenty of customers with automatic in particular that keep stock 2.5 cams because the resulting power curve compliments the shift program of the automatic that tends to keep the car in that low/mid rpms anyway. An automatic owner in particular will often intentionally stay with stock 2.5 cams because they want peak torque right in the middle of the range their transmission is programmed to operate (when not using the paddles and/or heavy right foot to pin the rpms higher).