With Greddy Cat-back you loose low-end torque.
Cat-Back does breath better, than Axle back, cus thicker piping and less air restriction from stock pipes and cat. but Does it mean faster? no
There is such thing as Back-pressure, loosing too much back pressure will have lost in low-end torque..... beside our Dual Exhaust is fake, we only have single cat.
I heard the new RSX-R has some kind of Exhaust Value, which closes the pipe during low end, and opening it up on high end.
which eliminates the low end lost from the stock 4inches tip.
Well, I drove my friend 's with Greddy Evo Cat-back, I laugh at him when my stock car beat him, cus he only changed the exhaust.
Now, after we all had bolt-on, header/intake/exhaust
mine is Comptech Axle, his is Greddy EVO Cat
I am still beating him from start. he might have more gain at high end, but 4hps more on high end doesn't made much difference, and you already lost 0-60mph.
Just from experience, Cat-Back isn't worth for the bolt-on. It may run better with Turbo/SC.
unless you are going thru Single straight titanium piping and exhaust. loose weight, gain hps.