I run SX with a Midex8 interface and don't have any timing issues. It doesn't seem any better or worse than VST 5.1, I had great results with that as well.
What problems did you have with VST before? And how did you test the system to "see" the difference?
Well feed a synth with a 32 steps pattern and increase the tempo- in cubase u can hear changes -just compare the output of cubase with a "light" sequenser like seq303- i 'm using logik 3.7 (Logik pro audio Isis) in a pentium II and the timing is perfect...
Also cubase doesn't record well the glide or overlapping lenght of notes...