Well, there are multiple variables here. First, it's not like there are any other cars we can readily explore on the market that use a similar NN on a beefier hardware. Do they mean the in-car HW is a lot less capable than some desktop solutions developers use? I can certainly believe that.
Reportedly during the call Elon mentioned that they think current HW can get about same safety as a typical human or some such which is well below the previously promised benchmarks (around 27ish minute).
The only running NN right now uses about 20-25% of the existing GPU. Of course more NNs would use more, how much more? A bit harder to tell, but we can see the rain-detecting one is really small and we don't need to run it at 30fps so unlikely to have a big impact. The repeaters one is about the same size as the front looking one, but we need 2x as much power because we need double the cams (they apparently intend to run this net for both repeater-cams and the B-pillar ones).