We just went on the Backlot Tour this past Saturday. They now explain what the new construction is. It is indeed "Lights, Motors, Action!" They say that it is "based on" the stunt show in Paris.
In the "costuming" tunnel they have some motorcycles and cars that will be used for the show. Also, outside at the beginning of the tour, there was a truck with a flatbed trailer carrying two of the little black cars (covered). WDWMagic says they are GM cars. They are cool looking, but tiny!
Most of the custstructions appears to be done. In fact, I do not think they'd get this many cars in, if there was still a lot of construction left.
I would guess that soon, they will be working on the show, rather than the venue.
Ted