The P-515 is very good for what it costs. I tried a whole bunch of them before deciding, Nord Stage 3 wasn't as good at almost 3 times the price. And what it lacks in gimmicks you can easily add in software.
I get so much entertainment out of just switching between the default Yamaha CFX voice and the Bosendorfer. They make the same song sound so different!
I'm excited to give Pianojacq a try soon! Maybe I'll even try my hand at custom user CSS. :) Have you given any thought to a theming engine?
No, a theming engine is not even on the list of 'possibles', but if you want to add it there will be a point when it would be a good moment, first the UI needs to stabilize.
Note that it works perfectly if you download it so you could download it and patch the CSS if you want to do this more permanently (you'd have to rebase periodically if you want to keep up with my development).