Supersprint was able to do exactly that with their stepped design headers plus associated exhaust pieces. On the dynograph below the thin line is the baseilne with stock headers, cats, and a Supersprint rear section. The thicker line shows the stepped headers with 100 cell cats and the Supersprint rear section.
Peak power gains are ~15 horsepower in the 6200-7100 rpm range. 18 horsepower is gained at 7700 rpm. Down low there is torque gain from 2200 rpm to 3400 rpm. After 3400 rpm the curves are even and there is a slight loss from 4000 to 4800 so there is a tradeoff. The tradeoff is worth it for the gains down low and up top.
Combine this with a good tune and that slight dip may even be eliminated meaning gains through the whole curve and more power for the highly strung GT3 3.6 liter flat-6. Great to see Supersprint produce parts for this motor.