I am both a musician and a geek and I've been there - i do my arranging and the playback from an arranging program (Finale/Sibelius) is pretty sophisticated these days. Software + synth will replace an average performance pretty well, but a great performance is great because it pushes the limits of the players and the environment. A great performance requires great individual performances and will be on the risky side. Typically a great performance (I've occasionally been lucky and been there, great performances are very rare) works this way: you are playing along and someone, perhaps the conductor, perhaps not, makes a proposal: "Lets go for it on this one"" and plays a stunningly great phrase. The proposal is answered by another wonderful phrase, and from then on to the end (if it goes right) everybody is concentrating at a rare level. I've occasionally heard performances that start great & finish ordinary, too.
Want a couple of examples? These are from the classical area but I've heard it happen in jazz too. Try:
Leonard Bernstein+NY Phil, Mahler Symphony #2, the version with Lee Venora singing. The brass playing is superb too.
Same conductor & orchestra, Sibelius Symphony #2, The great phrase is the oboe solo.
What we have now is software that gives us the most of the nuances and produces a polished copy of a fine performance. I can imagine software that would give us true greatness but not any time soon.