I agree...the MSP Miata and RX-8 will be two very different cars...Even if the Miata ends up being faster, lighter, and a better handler...people looking at the RX-8 will probably still want an RX-8 over the Miata...
Porsche is running into the same thing...There are rumors and specs of a new 928 coming out by the end of 2004, and a new version of the 911...Both are speculated to possibly use a similar V-8 engine (this will through the 911 community into a s*** throwing contest, remember what happened a few years ago when it merely got a water cooled engine?)...the 928 will have a base price of around a moderately equipped 911, be slightly more powerful as well...And everyone at the Porsche forums are saying that "no one will buy a 928, it won't be as fast or as light as the new 911"...but IMO that isn't the point...the 928 will be a GT car with more room, luxury, and a softer suspension...and the 911 a sports car with less of all of that...
The RX-8 will clearly be bigger than the Miata physically, and has adequate room for kids in the back (if they are a year old)...and is therefor in a completely different buyers market...If people were only worried about performance, they would ditch both ideas and buy an EVO...