All of those instruments you mention are ones that have CONSTANT inflection. In other words, to sound realistic you have to use TWO hands to get the bends, trills, scoops, doits, moans, etc. from them. This is a real problem if your left hand is tied up doing the chord input constantly.
Not to mention that brass voicing is a two hand thing... look at any bigband score, or EWF or BST transcription, and you will see intervals you can't play with just one hand! Likewise strings, guitars, many things...
The truth is, the sounds you already have, if played two handedly, can already sound utterly convincing (check out the factory demos if you don't believe me!) but a better 'sound', still played in a one handed manner, will get you very little closer to where you want to be.
The cheapest solution to your problem? Prepare sequences for the songs you want to use these sounds on, then use BOTH hands to get the inflections you want, on the sounds you already have! Practice, practice, practice... getting all those inflections right isn't intuitive! It takes a lot of work, but once you get it down, you'll see the benefit of it, AND save yourself a chunk of cash chasing something that can't be done (at least in normal arranger mode)

[This message has been edited by Diki (edited 12-31-2008).]