Sample layer switching (Which is what SA is) has been used in the pro market for years, (Just look in any VST sampler) however these days most have moved towards sound modelling, which means the sound is created in real-time, just like the real instrument.

The reason the T4 SA sounds so good, (Although I have yet to find anybody outside of arranger players that rate the SA Saxes) is the meticulous programing by the designers, for the market it is aimed at. (The Home User)

Bill