Does the RX8 idle shake a little?
 

not sure if this is normal since I dont drive my car too often
but the needle holds perfectly fine and the car make the brrr noise and the engine does not shake at all
but every once in a while there will be a little bump or hiccup, the needle doesnt move but you can hear it

the car has a few mods so I am not sure how normal this is
D585 coil packs
BHR midpipe with ceramic cat

My 04 does the same thing. My car have some vibration at idle at times but it does need engine mounts. I wouldn't be terribly concerned personally.

Those mods shouldn't cause any excessive shaking, it should have been clear if the coils were hooked up wrong.

Depending on mileage if your engine is lower in compression, if the idle dips it may run rough and shake(usually combines with older engine mounts). Otherwise next time you venture under the hood you could always just check all the intake gaskets and vacuum elbows. Not a bad idea to clean the maf and the position sensor as well.

Just like the others mentioned it may be motor mount issue, I have a mount that the center pulled out of and replacement on the way. Occasionally I will get a sleight vibration due to it