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Items here are not directly related to power. Drivetrain items that are related to power are listed above in the Power section.
This doesn't "Add power", but rather "changes the intertia dynamics of the drivetrain." Two cars that are otherwise identical will dyno the same numbers (on the same dyno at the same time) with a difference in flywheel weight. However, the engine will spin up, and down, faster in the car with the lighter flywheel, which translates to a more responsive engine and (potentially) quicker acceleration. Quicker acceleration means "more power" to most people, but this is an exception to that. The engine isn't making any more power, it's just having a bit less difficulty in reaching redline, so it gets there faster on the same amount of power.
Just having "a lightweight flywheel" isn't going to mean much though. Not all 11lb flywheels are the same in their overall effect. The lightweight 11lb flywheel with the weight removed from the outter edge of the flywheel will spin up and down faster than the lightweight 11lb flywheel with the weight removed from the center of the flywheel. Most companies know this, and go as far toward the outside edge as possible, but they all succeed in different ways that are bound to create differences between the flywheels, even if they have the same end weight.
The normal drawbacks are the expected drawbacks of when higher inertia is beneficial, such as how quickly the car tries to slow down when moving to an uphill, or starting from a stop. These are usually non-issues to most people, though if your manual transmission skill is lacking, you may want to consider postponing the change until your skill improves, as you will be more prone to stalling when coming to a stop or starting from a stop.
More technical reading here: Stephenmason.com: Rotational Inertia
The OEM clutch can handle quite a bit more torque than the N/A Renesis can throw at it, so upgrading the clutch is often only going to impact your clutch feel. Hopefully in a direction you prefer. If you go to turbocharging or supercharging, then clutch selection can easily become more of an issue.
Warning: Exedy Stage1 clutches are becoming increasingly suspect on the RX-8. Vendor reports, plus several members here (including me) are having problems with proper clutch engagement, strange noises, and premature failure. The suspected reason is an inconsistent 'stack height', resulting in improper clearances when disengaged.
There are no real publicly noted instances of people running a different transmission, though it's certainly possible. The 2 common swaps are between the series 1 transmission and series 2 transmission. Different gearing, reverse is in a different spot (next to 1st gear vs next to 6th), and differing internal strength are the usual reasons, series 2 being considering the "superior" usually, though there are exceptions there as well.
There are no real "driveshaft mods", however automatic transmission equipped cars do not have a carbon fiber driveshaft like the manuals. Be very wary of buying salvaged driveshafts however. Carbon fiber does not react well to shock that may have been delivered in a crash, and it could be a driveshaft just waiting to shatter. If you are removing the carbon fiber driveshaft for a different modification, be VERY wary of dropping it or banging it. Cradle it carefully and if you have to store it for a bit, make sure it's protected. You don't want to be buying a new one because a moment of inattention made it roll off the work bench.
The common mod is to swap gearing between series 1 and series 2, 4.444 (S1) vs 4.777 (S2). There are some aftermarket LSDs as well though, which are occasionally used.
Caution: If your RX-8 sees extended track duty, consider adding a rear diff cooler, as the OEM unit can boil the fluid.
Axles are largely also unmodded. The only exceptions are the really high power cars, typically after engine swaps or full race packages. The stock axles are plenty strong enough for anything less.
Note: Series 1 and Series 2 automatic transmission axles are the same, however the axles for the manual transmission RX-8s changed from Series 1 to Series 2
Following much the same concept in the Suspension Bushings, upgrading the diff bushings will stabilize the diff's movement more when you are trying to put power down. If you are experiencing wheel hop on acceleration, this is most likely because of rear diff bushing failure, or the particular tire compound you are running is overwhelming the good bushings.
This follows the same concept as the other Bushing items in terms of material, although with the size and shape of the motor mounts then it's possible to take an existing set of mounts and fill them with the substance of your choice to firm them up to the degree that you are looking for. More than any of the other bushing items though, the motor mounts can easily impact quite a few things, most noteably transmission gearing selection. The sloppier your motor mounts are, the harder of a time it can be to get the car into the desired gear, usually starting with 4th gear. Not a universal statement, but quite common.