This is what exactly happened to me when I put the high flow Downey muffler on, after the headers and the highflow cat, and the exhaust valves are stock. I have plenty of power at 3200 and above, but the low end torque is gone.

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>>>*Yes, exactly the risk with a too free flowing system on any vehicle.
Likely the muffler complimented the header, and moved the powerband higher.
This is why I personally feel that some intake flow must be added, this does help some with that.
One could advance the camshafts a couple of degrees, perhaps. The computer might not like that much, though.
It would be easier to put manifolds back on and keep the pipe and muffler setup.
Restricting in the pipe and muffler is the wrong place to create backpressure....It does have that effect, but it also affects mid and top end...Tricky stuff with modern engines....*EB