Sorry, Fran... they ARE separate sounds. The Superior and Classic are NOTHING like each other. But they do have a common direction, rather than one being brittle and thin and another being warm. They ALL are fairly warm, at least compared to Yamaha...
It's fairly easy to test. Start playing an arpeggiated figure or some repeated chord, and switch between the pianos as you play... If you hear more than a simple EQ change, it's a different piano. The Sonic Cell has the same pianos, and to my ears they were NOTICEABLY different.
The E60 patchlist shows all three... Natural, Classic (the G70 one) and Superior. How much compromise in the sound between the E60 version and the E80 or Fantom versions, you'll have to listen for yourself. It's tricky to tell from demos, but some of them are up at PurgatoryCreek.com (the apples to apples piano comparison site)
BUT... no, they aren't the same as the RD-300GX. At least, probably not as good, and I know some are different (because there are some rumblings about sample matching and wolf notes on the 300GX and FantomG from FantomX users)
There is an SRX board, but you probably need to compare it to the pianos already in the Juno Stage, to make sure it isn't already in it! (that new Rhodes SRX board sounds good!)