I have StyleWorks XT and I've messed with conversions. IMO Yamaha styles don't convert "faithfully" to Roland, probably because there are too many style control parameters that Yamaha, Korg, and even Casio (!) have, but Rolands lack. My solution was to let the Roland decode my key presses, sending full triads and tetrads to a Yamaha via MIDI. Ultimately I decided that I could live with the Roland styles for the Pop / Singer-songwriter stuff I like to play.
It's really a shame that Yamaha never made a module. One of these days I'm going to build the worlds smallest dual-brand arranger with a Fatar-49 controller, BK-7m and a QY-100!