I was at Universal Hollywood a couple weeks ago - and you can clearly see down and into the construction zone.
But given the footprints Singapore / Hollywood and even Osaka have to deal with, just kind of wrap your heads around it a little bit considering they need room for ride buildings.
Attractions Magazine just posted some new photos from this weekend:
Total Hollywood Footprint:
Here's a closer look at the Mario Kart "track"
While the Hollywood land should be smaller than Osaka / Orlando, the actual Mario Kart footprint should be pretty similar. The ride itself seems to take up a similar amount of space that Transformers does - though Transformers obviously stops in front of screens for big periods. I'm assuming Mario Kart is going to use similar tricks to "extend" its track length.
No screens! Also the ride will be over two levels I believe, like Transformers.