So he's got an RX-7TT that has exhaust and he's getting an ECU? Run him. If you hook up some N20, you're gonna beat him. He will not have 300rwhp. RX-7's lose more power through their driveline than your Mustang. It's IRS is pretty inefficient, and you have to know how to launch it to get really good times. The cars are pretty weak in the mid/upper range, like 80-120 from the factory.
Stock RX-7's weigh a whopping 300lbs less than you
He'd be lucky to have 250rwhp. If you run a 125-150hp shot of N20 and he gets a bad launch or you run him from a roll, you're gonna win again.