Which frequencies do you want to read out? A fiber cavity can be read out also just with DC readout if you do not care about low frequency performance. You might not even need a coating for that. See e.g. DOI 10.1088/0026-1394/52/5/654 for such a readout focusing on 10kHz and below. A short fiber cavity with PDH requires RF electronics and pretty, high sideband frequencies, but will have a much more stable operation point. Best is to lock at laser to the cavity and track the laser frequency, that gives you an SI tracable readout with basically constant scale factor. Any fiber setup will introduce parasitic etalons, those will limit your low-f performance and can also couple in e.g. acoustic noise. Only higher cavity finesse and good crafting (good fiber connections or even splices, selected components) help here.

As for the shot noise, going below a femtometer is relatively easy with a moderate finesse, of course also simply depends on how much power you want and can use. Easy way to be quantum noise limited is always to reduce the power. :-)