I know when I installed one in my 97 I noticed an insane increase in speed... not so much HP or torque, but hearing that whistle just made things seem to fly by so much faster than before.
I took mine out and gave it to a buddy, I didn't notice any increase in mpg, or HP... not sure what effect it would have on a Renix, but there is only one sure way to find out.
At $27, even at 1% increase in mpg its paid off in just a couple dozen tanks.