I'm not a pro, just a hobbyist for the last 20 years or so.
I've literally had dozen's of workstations, synths, samplers & keyboards.
Wasted many years trying to get better sounds for my keyboards by adding various modules. I got caught up in the technology rather than the music.

To me it makes sense what you're trying to do. Use the keyboard to create your compositions, then use East West as substitute for sounds. I'd really think hard about putting the sd2 into the mix.
I doubt the sound quality of the sd2 would match the quality of software samples.
Maybe with a keyboard that has quality sounds, you'd be able to leave some of the style tracks in your final mix instead of replacing the lot with samples.
The yamaha's have the mega voices, korg has rx voices ( velocity switched), plus in guitar tracks you get fret noises etc. SD2 won't replicate mega or rx voices.

I hadn't fully realized what the korg rx voices were. I used to happily increase the velocity of a track, when converting a psr style to korg. PSR tracks are recorded at a much lower velocity than korg styles.
Eventually realized I shouldn't use RX sounds in a style, unless the style is recorded from scratch.
I'd used some sort of an RX funk guitar in the conversion. Didn't realize that a velocity of just say 40 triggered a totally different sound to velocity of just say 60 & sound changed again at an even higher velocity.
If that RX sound had been used in an onboard korg style, & I played that style via the sd2, it would have sounded all wrong. To me, some style tracks are best played by the keyboard they were written for, unless you decide to rerecord them in the actual composition or rerecord the style track to suit the soundsource.
( Korg style editing functions are really good)

Anyway good luck with your keyboard quest.

best wishes

[This message has been edited by rikkisbears (edited 07-18-2009).]
best wishes
Rikki 🧸

SX900, P121 Piano, Band in a Box 2020
V Arranger/Ketron SD2