it's two fold - Futuremark need to update the system to accept the new cards, usually takes a few weeks to resolve, and yes it's likely that the drivers are not FM approved so will not be entered into hall of fame.
Sill valid for Mbot though mate, providing you post a screenie of the score achieved. A good way to see if the cards have the same block fittings is to swap teh 7800 air blocks with teh 7900 to see if they fit, I am confident they do.
Mav