See my post on the Hemi dyno motor tests I did last month
That was not in the car but the 3.0 inch head pipes with a set of 3.0 in and out by 14 inch long straight through Ultraflows did make decent power on the DTS engine dyno
I have tested my old pump gas Duster with a full three inch system on it with 14x7x3.0 in and out Magnaflow muffs mounted at the rear bumper versus removing the complete exhaust system at the track, .02 ET and .2 MPH difference in the 1/8 mile 7.01 ET at 95.2 MPH with the exhaust sytem hooked up and 6.98 ET at 95.4 MPH with no exhaust system on the car, Hooker A body fender well headers with 2.0 primary pipes and 3.5 inch collectors
I think the weight was possibly part of the gain and the lack of back pressure was a small part also, not enough to remove it again though