Theoretically, the only reason the RX7 redlines at 9000 rpm is because of the accessory belts.
A stock 13B will go to 15k before the heat and inertia are too much, and it begins to warp. Without accessories, 12k is a reasonable redline for it.
The rotary will go as fast you can put fuel pressure into it - At some point it expands, the apex seals compress, and then it's metal-on-metal for about .00004 second before it detonates like a grenade.
-> Steve