Who cares if it's a race header? You'll get an extra 2-3HP from a 4-2-1 compared to a 4-1, but you'll have a lot more torque on the 4-1 Comptech design versus something comparable to DC. Torque on an RSX-S is good, top end is already fine.

Power curve > peak numbers

And that flange looks like it's made to hookup to the stock flex/cat.