I had switches from more sources: NIB modern blacks, heavily used modern blacks, vintage blacks from more sources and with different amounts of wear. Unfortunately, due to lack of equipment and time at the time, I only tried to swap different parts and did some blind testing consequently, but I couldn't document the whole thing properly. The feel depended on plastic parts used, not springs, nor lubing (none). It was consistent across batches, i.e. independent on use. Aging by itself could affect feel, but more batches were made around the same time.
Once I have more time, I'd like to bring some switches and keycaps to a lab.