Well I have a G6 (slot-2) and just ordered a G6Real.
As far as I know from reviews and comments from people owning a G6Real, the rom compatibility is 100%, and all running at perfect speed.
I think I read download play compatibility is very good but not 100%.
And the last thing you could say that's not so cool about the G6 is that the DLDI patch for homebrews is not yet perfect but it was just released.
As for saves converting, I'm pretty much sure it's possible as I remember doing it for my G6 (slot-2) a while ago and the save type for G6 (slot-2) and G6 Real is the same (.0, .1, .2 => you can have three savefiles for a game).
I don't remember if I just renamed it or used a software (was more than a year ago when I messed up a savefile and downloaded one to replace it)... So you'll need to do a research for that!