Hi Diki,
I don't think there's a perfect soundsource for playing the Yamaha megavoice styles. Whatever sound source you use , the styles are going to require a certain amount of editing.
Also you may want to check into the drums.

I still use OMB software as an editor for for my style conversions across to korg format. Means I don't have to do my conversions using EMC.
I'm not very knowledgable as far as sounds go, but it was recently pointed out tto me the RX sounds in my PA800 use velocity switching. So it will be interesting to try & work out how they compare to to the megavoices. The RX sounds also have fret & other noises that can be used in a Korg guitar track. IFrom memory, and I haven't done it in a while, but I think I had to transpose some of the sounds. If the velocity switch RX sounds work are similar in the Korg, it may just mean altering the velocity of some of the notes to suit the RX sounds.
I have to dig a bit further into that before I'll be able to tell if it will work.

Ive downloaded the new demo, but haven't had a chance to try it yet.

best wishes
Rikki