The sound comes from your expander. So, if you would use the roland to control your yamaha, the sound would be louder too... Another thing to consider is the amp and speakers you play with. And the roland has alot of different piano sounds. And ofcourse the fact that from a certain level, you just notice the difference between the real thing and any digital paino. (The velocity doesn't change the timbre, the pedal doesn't allow other strings to resonate...)
But the fatar I played really sucked. But I don't remember what model it was, could be pretty old. (Was used...)
The new rolands have a different action too. Never really played it.
Guess it also depends what you're used too. Real piano's have different feels too. I remember as a kid, always playing uprights, at my lessons an upright with VERY heavy touch, and on our examination we had to play a yamaha babygrand, with extremely light touch...