SNP keys have been built, and were apparently described in Mars 2004 QST. As I'm not a member of ARRL I can't access it, but the first page of the article can bee seen in the following video:
https://www.youtube.com/embed/wsE-S2eXhK0https://www.youtube.com/watch?v=wsE-S2eXhK0
It says that "experienced users routinely reach 25 wpm or more", which could be interpreted as being comparable with a paddle key. They do continue with saying that "for most of us the puff-and-sip key will never quite match the speed and convenience of paddles", but this could mean various things. If the comparison concerns an operator with long experience with paddles and none with SNP, it doesn't necessarily reflect negatively on the latter.
In the video, the author demonstrates a straight key version, with only one switch. He then notes that he can send faster than with a normal straight key! He only uses blowing to actuate the key, but his contraption seems to use Bernoulli's principle (?) and should work for inhaling as well. If that's correct, and if the switch is fast enough, it should be possible to operate it like a SNP cootie key!
All in all, it seems like SNP systems can be (at least) good enough to make full speed QSO's.