I don’t think the final incarnation of the MK plans have it overlapping itself. However, the queue is on the upper floor and entire ride is on the lower level.It does look like a small amount of space, but I think they will use vertical space to give the Mario Kart ride a decent ride length. I think that the track might pass over and under itself a lot. The Mummy is in a building that isn't very tall, so it doesn't have much vertical space to work with. Since Mario Kart is being built from the ground up, there shouldn't be any major space limitations preventing it from being a long ride.
Mummy is a roller coaster which takes up a lot of room, and you travel through much of it at a fast speed. MK is a busbar dark ride that moves slowly. I would compare it to something like ET instead, which the MK building is a similar size. But, unlike ET, it doesn’t have to share the space with the queue and preshow, since it has two levels.