Oh sorry, I didn't read your posting carefully. Well the Megavoices makes the sound of guitars more natural, good noticable for all kinds of "unplugged" music. But in other kinds of music, like big band swing, the Mega Voice Technology makes no difference.
But some voices are better too, especially the acoustic steel string guitars, a litlle better than camp fire guitar. And the medium sax voices are better too, now you have mellow sax ensemble (for 40s Swing), bright sax ensemble (for Smoke Gets In Your Eyes in the Ray Conniff version)
The availability of optimized styles is not so good. D-o-o and Hage offers Tyros styles with Mega Voices and Yamaha itself also offers some Tyros styles.
Here you will find free Tyros downloads http://heidruns-musikerseiten.de/tyros/styles.php
About the better voices, it could make more sense to add a Roland tone generator for some better voices.