Dynomax aftermarket exhaust issues, back pressure gone=low torqu

hey everybody this is my first post here. I recently began doing some modifications on my 02 celica gts. i started of with the ecu and knock sensor conversion, a Cold Air Intake, a short shift kit, and the now problem child the exhaust. I have 2.5 inch pipe all the way back from the headers to a dynomax supertrubo muffler. the superturbo is the most free flow exhaust dynomax is currently making The shop informed me.
So this is what happened. i went from the stock 2.25 in piping, cat, muffler, and resonator to a 2.5 straight through to a high flow muffler.
the results: great upper end. a very deep loud exhaust(to loud for me) and..... a complete loss of low end torque. To everyone who says back pressure is a myth... you are misinformed with our cars.

so my question is what is the best way to restore some low end torque and back pressure?
add a standard flow 2.5 in cat? add a second muffler close to where the cat should be? please help. im at a loss of what to do next here.

PS, CEL will not activate due to the fact the engine ecu is the JDM 190 horse rated computer that did not have emission standards and does not read the second o2 sensor down stream of the cat.