MWJB Thought provoking great post. It forced me to think through this to organise my long rant as follows:
Grinder quality may affect filter coffee quality/flavour only up to a price point, after which the returns diminish rapidly. The price threshold is most likely lower. You have lots of data to establish the price sensitivity.
Grinders impact filter coffee through particle size + distribution and fines say under 400 microns.
Are differences in particle size distribution vary widely in poor / cheaper grinders ? If so, they can impact the sensory aspect. I am thinking good grinders unlikely to suffer from this (PSD), besides bringing additional clarity, smoothness, repeatability and sweetness to the coffees. I think (large) flats may also help here. While travelling, I find the filter brew using JX Pro vs Duo filter burrs differ by clarity and smoothness on the same coffee. So is using switch vs clever as hybrid.
Once fines are controlled and median size is stable, further improvements in PSD have diminishing sensory impact for filter brewing. It is therefore not surprising that brews like 9% 400 Kruve and 12% 400 Kruve have been consistently producing good cups.
Filter is more forgiving than espresso with cheaper grinders if a brewer is fairly comfortable to adjust brew parameters.
For filter brewing, we know the grind size selection, water chemistry, brew ratio, coffee solubility, filter paper, and brewer flow dynamics exert greater sensory influence than just grinder once baseline grind quality is met.
Experienced tasters, through blind cupping, should be able to pick good vs other wise cup between poor and good grinders. The chase for expensive grinders is both personal and influenced.
Differences in personal experiences may be explained by sensory acuity, palate preference, brewing, and brewing than grinder quality alone.
I think sensory curve performance can be broken into three as a function of grinder quality and not necessarily by their price.
Sub-Par grinders → broad PSD, excess fines, bitterness, dryness, muddled cups (improvement obvious to all).
Good hand & mid‑tier electrics: stable PSD, controlled fines, clean flavour; differences are small and preference‑dependent (many may get max enjoyment).
High‑end large flats/precision builds: narrow PSD, high repeatability, subtle clarity gains, often only for trained tasters and may not be universally preferred.
In summary, grinder quality matters but reaches steady state quickly for filter coffee. I don’t think there is a universal grinder price point at which flavour enjoyment suddenly improves.
I recommend that you create another thread for espresso. I think we are likely to see a lot more responses than for filter. 😊