Being pedantic about staying in arranger mode is what kills the realism, IMO.

Make the arranger version FIRST, then start to take out what you CAN play better, and leave in what you can't.

Also, as we have heard with a few really good home organ/arranger demos, using the swell pedal on different sections can add in an enormous amount of dynamics, which is one area that straight style playing can miss out on.

You can also mess around a lot more with changing sounds for different sections, counter lines (tough for an arranger to get those right), hocketing, and many other traditional orchestration techniques.

The Yamaha does a fine job of starting you on the journey, but, as already stated, you can already HEAR that there IS a difference, and carrying on down the path by using the sequencer to polish and hone will take you even further.

If you aren't computerphobic, you might take a look at something inexpensive in the way of an orchestral library like Garritan Personal Orchestra, to either double or replace certain sections of the Yamaha's orchestra. Up bow/down bow round robin triggering (that many of them do) can add quite a bit more realism to the production. And, if you can get the latency low enough, even allow you to play these sounds live from the keyboard...

The Yamaha MIGHT be about the best there is currently to 'jam' live with an orchestral sound. But there's still a LONG way to go..!