a. it's not performing circuit build tests
It's unclear what you mean here. The scanner has to build circuits to measure their speed.
I've got this impression that torflow doesn't try all 2 hop circuit permutations... but maybe I'm wrong about that, i haven't checked yet.
b. it's not distributed and thus more easily gameable
Yes. But this is a major change in design and implementation.
I'm hoping the Peer Flow design becomes a priority to implement. So far there is no spec or tor prop type document.
Yes and getting even more off topic, this latest mixnet paper talks about using the Directory Authority system to report and vote about link performance to mitigate n-1 attacks:
[MIRANDA] Leibowitz, H., Piotrowska, A., Danezis, G., Herzberg, A., 2017, "No right to ramain silent: Isolating Malicious Mixes"
Instead, we could increase the circuit build failure penalty in the current bandwidth authority code. It was originally designed for a much smaller network, with different load characteristics.