I noticed a torque difference in my truck from just replacing the stock muffler with a high flow muffler. . . . And Yes I did look into the resonators when they removed them to see what they looked like.
Your TRUCK? Sure, there are many other vehicles, especially non-turbos with pretty restrictive mufflers. There can be some gains by going with a high flow muffler.
But our stock muffler is just about as high flow as a muffler can possibly be by design. Our stock exhaust system, including both resos and the muffler will flow more cubic feet per minute of exhaust than our engine is capable of taking in on the intake side, at least untll we get above 300 whp.
And those resos are straight, aren't they? No curves, no changes in tubing diameter, no bends, and looking through them is like looking through a wide open 2.8 inch pipe.
I actually added a third reso to tame drone after going competely catless with DP/RP. I haven't dyno's this engine. I do run 30-100 mph tests before and after changes. I can tell you that adding that third reso did not change those times one bit and they are pretty good times: 10.1 seconds, consistently both with a stopwatch and with a GTech Pro accelerometer.
So, add or delete resos and mufflers however you want your car to sound, but we have an incredibly freeflowing exhaust system from the factory. Resos change sound. With respect, by design, they do not change flow.