I have measured 150+ of the W18s and they are pretty darn close. The Fs is extremley accurate. It hovers from 30-33hz. The Qts hovers from .38 - .42, and the VAS is about 30 liters. I must admit that my measurement jig is quite tedious at times, and that I keep an old driver for reference.
Unfortunately, I cannot provide solid "statistics" as I have not maintained a database of drivers.
Probably the only significant W18 change I have found over time is a slight change in the roll-off shape around 2700hz a couple years ago. A new batch of drivers measured consistenly about 2db higher at 2700hz. This wasn't terrible, and luckily corresponded with the redesign of the 1801b crossover. I learned that slight changes in the rubber surround material will impact the shape of the driver's natural roll-off. The rubber surround material impacted the roll-off.
Overall, I am very pleased with the QC of the SEAS W18 woofers. I have only sent 1-2 back for scratchy voice coils. This is very minor IMO. When this happened SEAS was VERY interested and very eager to research and fix the problems. They are a very solid company.