Even though the Mazda RX7 is very powerful in its standard form it is very easy to tune and squeeze out a lot more power. There are many examples of FD's out there with increased performance because 400+ bhp is easily achievable. These are the normal steps a tuner would make to see these kinds of gains.
An ECU is a good base to start which allows you to 'map' the way the engine works. This is also a good way to make the car more reliable working in safer limits.
Replacing the standard exhaust and air intake combined with a re-map using an aftermarket standalone ECU such as the Apexi FC Commander will open up the range a bit further for the garage who will be mapping your car.
Increasing the size of the injectors means that higher amounts of petrol can be injected into the engine. When I fitted the Apexi FC Commander ECU to my FD the tuner informed me that my injectors were fouling the safe limits so I had to upgrade the primary injectors up to the same higher capacity as the secondary injectors.