Back pressure doesn't create torque. You probably have more torque, but at a little higher RPM so it doesn't feel as torquey, but it is faster. I'll give you a personal example. My 00 SS had true duals. I had it flanged so I could run them dumped or out the back. With the back section on it felt like it pulled harder, but without it the car would consistantly run .2 sec quicker ET's in the 1/8 mile.
Buttdynos are notoriously inaccurate and I'd rather have a faster car than one that feels faster but runs slower.