R and X just look and sound good next to each other. I'm sure its one of those subconcious association things that marketing people spend a lot of time and energy on. Some combinations, for whatever reason, always put a subtle mental suggestion into the minds of the perspective buyer.
"R" is most often associated with "Race", so it makes sense on a sporting model. "X" has an excellent association with the word "eXperimental". People love the idea of the experimental prototype, because its always a leap ahead of what came before. Its new, its eXciting. Add the two together and you have a potent combination.
The letter "S" is an often used letter because it suggests both the word "sport" and "special". S seems to be a powerful letter in marketingland, especially to Germans. Consider that Porsche uses the S to denote "super", and Porsches with the S tag always make more power than the non-S cars. S is also the letter Mercedes chose to begin the name of their flagship car. When used in combination with another powerful letter like "R" to denote "Renn Sport", you have the legendary "RS" Porsche and Audi models.
I'm also of the notion that the later letters in the alphabet carry more weight than the earlier ones. I'm not sure why, but I think it has something to do with the notion that the later you go, the better.
"Z" is an excellent example of this. The Zed is used in a lot of cars; 350Z, Z4, Z28... etc. I guess its because Z, by virtue of being at the end of the alphabet, feels like the ultimate. I mean, after Z, there's nowhere left to go... so anything carrying a Z has to have an imposing sense of finality to it.
You can probably analyize everything letter to death --and I'm sure marketing types already have.
///M-Spec