There is also a thread briefly describing the 400RS vs the 360S2. As far as the difference between the "S" and "RS", I am not sure what you mean; there were amps like the 100S and 200S... is that what you mean? The 360S2 is not an "S" series amp, it's just the 360 Series 2. All in all, yes the improvement is substantial, and you will find bleeding-edge sound and technology in the RS series - in fact, I had non-audiophile (though music-lovers at heart) guests in last night (and one of them is a musician), and they were pushing each other to sit at the sweet spot, playing analog of course. This hasn't happened before the new Spectral gear, though they still admired the sound back then. The reaction was overwhelmingly positive, especially when it came to timbral accuracy.
As far as the 300RS vs the 400RS goes, I have yet to compare them against each other, but my recollection is that they do sound a little different. I would strongly suggest an audition before you spend any money, assuming you can afford any of them.