You can most definitely make Yamaha keyboards sound orchestral! Amongst the things that I have in my repertoire, or have taught to students:
Massenet - Meditation from Thais
Handel - selections from The Water Music
Mendelssohn - Violin Concerto
Clarke - Trumpet Voluntary
And for 'souped up' Classics, one student played several Classical pieces for his Honours in Performance Diploma exam:
Mozart 40 - based on the 1970s Waldo de los Rios version, but extended with more of the original piece
Offenbach - the Overture from 'Orpheus in the Underworld' - his own arrangement
The Red Sarafan - a traditional Russian melody, arranged by me, with a nod to James Last
The Kino Strings, Seattle Strings, Woodwind solos and ensembles, Symphony Horns etc can be used and combined to create some very realistic orchestral colours. As for style use, you'll find that the Freeplay styles are excellent, though you'll need to revoice and rebalance them. And there will be some creative use of Sync Start and Sync Stop, as well as a lot of 'AI Fingered' chords to get the bass lines correct.
To really do the keyboard justice, the player has to have decent knowledge of orchestration, registration and arranging Classical material for arranger keyboard. And the skills to perform it.