Uhm... I hate to be the educator when it comes to Toyotas on a BMW forum, but it's late and my better judgment has gone to sleep.
The "X" label for Toyota was there initial sports designation, much like the "M" for BMWs; however, they only kept that designation in Japan. This tradition was started with the introduction of the first (US dubbed) Supra back in 1979. In 1978 and 1979-1981 the (US) Toyota Celica Supra was known as the Toyota Celica XX in japan. Ever since then, "X" has stood for supreme luxury and performance. Hence if you notice on Toyotas class distinction on individual cars, the HIGHEST luxury edition will have an "X" in its designation. I believe the use of the "X" as a symbol of power and luxury has something to do with the 1967 Toyota 2000GT; that was known as the "X car." The Supra, the 2000GT sucessor was supposed to be 'twice the car the 2000GT' was, hence The Toyota Celica XX. In the late 80s to mid 90s Toyota developed TRD (Toyota Racing Development) which pretty much became the mainstream "X" in the U.S.; but in Japan "X" is still the denotation.