I premix both of my rx7s, I use... hmmm not quite sure of the name but it's TWC3? anyways, i get it at O'Reilys/autozone out here. If I ever own an Rx8, especially the earlier years, I would definitely premix, I'm sure you could get them to last well over 150k miles if done early on, especially because of the oiling issues they had.
Only cons I see with premixing are just the extra time it takes at the pump, which really isn't that bad at all, considering it extends the engine's life considerably.