I guess a lot depends on the use case. In my case quantization was not a problem (the scores were "rough" and query independent anyway, so they did fall into corresponding buckets). "poor" performance would then have to be backed by what the requirement really is – if one needs sorting by exact scores then the method used to speed up FSTLookup simply isn't a good fit. Still, compared to fetching everything and resorting this is a hell of a lot faster, so many folks (including me) may find it helpful.
It all depends, in other words.
As for using more buckets – sure, you can do this. In fact, you can combine both approaches and use quantization to prefetch a buffer of matches, then collect outputs, sort and if this fills your desired number of results then there is no need to search any further because all other buckets will have lower scores (exact).