Old thread ressurection, sorry......but since it has some info about Euro spec, i will just add few more related :

4 port engines with 5speed MT do have different flywheel and 225mm clutch

6 port engines with 6 speed MT uses 240mm clutch (same disc as FC turbo and FD, same pressure plate as FC turbo)

however, 225 mm clutch has center (splines) same as 240mm clutch, so technically 6 speed might work on 5 speed cluch.....

And quite interestingly, 5 speed trans seems to be pretty much same as FC turbo/ FD trans, atleast main parts of it......

I have not compared 5 speed (4 port) and 6 speed (6 port) front counterweights, but if they are same, you can most likely put 6 speed flywheel (and 240mm clutch) on the 4 port engines....

It's the basically the same r5m tranny as the fd. The actual gearing is different though, and the bell housing is the same style as the FC. There are swappable parts to make a nice transmission to fit in the 8.

Do you have gearing specs for 5 speed from RX8?

i have done this swap on my car.i have a 5 speed 4 port JDM rx8,and had another one 6 speed car for parts,so...i changed the gear-box and that gave me more acceleration,but i have bigger fuel consumption on the highway at cruise speeds as the motor needs to have arround 3700 rpm for 120 km per hour,via 3200 witch i had with the five speed.anyway i am not concerned about fuel consumption as my car is turboed also,i have changed the rear differential but did not notice any difference,in the end i have changed the suspension witch is more better and little lower.why i did not buyed a 6 speed?i had one before and sold it,after some time i have found the five speed in really good condition and buyed it.Now is turboed and has gearbox,rear diff,ECU,clutch and flywheel,crankshaft,rotors,counterweight,shocks and rims from a 6 speed,so is not a five speed any more:)why i did not put a 6 port engine.I have one layng in the garrage but the programming for this one and all the money and soul i have put on it where perfect so i preferred to rebuild mine,and to be honestly 300 hp for me is more than enough....
P.S.i am not sure about the fd,but the 5 speed tranny is EXACTLY the same in the mx5 miata 1.8 liters,and another observation,i have been SOOOOO bad with my 5 speed but never had issues,on the 6 speed i managed to fuck up synchros 3 times,and the lay gear bearing is whiiring all f....n time......so now i am thinking to go back to 5 speed

GEARING EU MANUALS 5 Speed - 6 Speed
 

Maybe I am late with this but anyway:

GEARING RX-8 - EUROPEAN Manuals - 5Speed and 6Speed

Comparison of gearing specs for 5 and 6 speed (European 4Port Manual, 6Port Manual)

5-speed gearbox Standard Power
6-speed gearbox High Power
I. 3,48 3,76 II. 2,02 2,27 III. 1,48 1,65 IV. 1,0 1,19 V. 0,76 1,0 VI. – 0,84 R 3,29 3,56 Final Drive Ratio 4,44 4,44

Ať to jde :-) Interesting fact is that on the Highway the 5 speed drives with lower RPM, it is also slightly better driveable in city driving or more unpleasant conditions (there are some positives of 5 speed - but there are no special oem fit kits for it - like Stage II or III EXEDY etc.)...