i have an 88 GTu amongst my collection rite now, the exhaust on it is the racing beat road race headers into dual 2inch straight pipes then in to some straight through mufflers my buddy picked up for me when he was stationed in japan, i cant spell the name it's in japanese. It is freekin loud the cool part is it sound like stereo since the road race headers let each rotors exhaust port vent into its own piping then muffler. from experience my best advice would be to with the racing beat full exhaust for your specific model RX7.
3 inch is to big unless your going with single exhuast rather than dual as the stock is.
check out the systems on
WWW.RACINGBEAT.COM
the nice thing about racing beat is all the systems they have are dyno tested, and very acurate.
i got the turbo 2 complete exhaust for my 87 and they stated a 59 horsepower gain on a stock setup, i got a solid 64 horses when i dynoed and i didnt even change from my stock intake yet.