Not safe at all !! At the end the kids or whoever squeezed him/herself to get in the back needs to get out as fast as possible in emergency cases.

 

Just for reference, the 996 gt3 seats fit with simple mods to the sliders. The upright part that attaches to the seat needs spinning around 180 degrees. Then they're at the same height / angle as a 993 RS seat.

 

A few years ago there was a BMW webforum arranged drive in California that resulted in one car with fixed back seats ending up on its roof and catching fire. The backseat passenger was trapped and couldn't get out, they burned to death.