Hi.

You'll probably be happy with the G-70 as it is, but if piano is
the main goal, the SRX-11 Complete Piano would be a good choice?

Here is a demo page og all SRX:
http://www.rolandus.com/flash/demos/srx/

Myself, I had the SRX-07 Ultimate Keys, but honestly, except
for some of the SRX organs, I used the G-70 native sounds.

If it's worth it?
Maybe. If you're not happy with the G-70 as is and the sound
you seek are on a SRX board.
Guess you'll find some at eBay at reasonable prices.