Sorry to hear about the lack of power gains with your setup. We generally like to package the Helix downpipes in our Powerpackage #1, which includes Helix uppipe, downpipe, and cat-back exhaust system which yields a 30+WHP GAIN on all the cars we have tested on (with no additional modifications). Without one or more of these components you will probably not be seeing these gains... In terms of exhaust flow, the rule of "being as strong as the weakest link" applies.
As for the downpipes, the Helix downpipe is made separately of both RSR & Invidia. Having tested both, we feel that the Helix downpipe offers better clearance as well as smoother head design than these other two.
Please contact us for any questions! Thanks,