I have experimented with guitar strumming by "playing" guitar via Guitarism iPad app and sending the midi out to various guitar samples. It still didn't sound as realistic as with Guitarism's own sounds.
I think what you need is the typical hard friction sound of a guitar string that is often missing in soft guitar samples.
If you try the Roland SN-A strumming guitar, this is exactly what you notice when playing a chord versus single melody note. SN-A not only converts the simultaneous chord into a quick arpeggio, but also changes the "samples" to a more friction-sound sample.
Hope you can use this observation...