One thing I haven't seen listed here is the effect of temperature and humidity (and atmospheric pressure as well). This affects both the amount of force the wind imparts on the sails as well as your ability to remain comfortable onboard (i.e. do you have the gear / clothing to maintain your body temp and manual dexterity in 30kts of 50°F wind while it's drizzling rain?). These are all factors that need to be considered into your definition and approach to "heavy weather" sailing.
As temperatures drop the air becomes more dense, and as the humidity drops the air becomes more dense. Conversely as air temperatures and humidity levels increase, the air becomes less dense.
That means that a blast of dry cool northern air in the fall blowing 20kts is going to pack a lot more punch than a hot and humid sea breeze mid summer blowing the same 20kts.
For example, on the Bene 36.7 I race we will typically carry our class 155 genoa until 20kts in the summer being somewhat overpowered in the puffs and fully powered up in the lulls. However this time of year as the air temps are on average 20-30 degrees cooler than what we would see at the height of summer, we would normally reduce to the class 105 jib if it is over 16kts, because the same 155 genoa that we will carry in 18-20kts in the summer being only somewhat overpowered, becomes uncontrollable and straight dangerous above 16-18kts in the fall.
I've seen comments claiming that the difference between 20kts in the winter (i.e. frostbiting) and 20kts in the summer can be as much as 20% difference in actual force being imparted on the sails.
lift = 0.5 x air density x wing area x coefficient of lift x velocity squared
Air density is essentially the sum of 3 variables, temperature, barometric pressure, and humidity.
https://en.wikipedia.org/wiki/Density_of_air
someone smarter than I can plug all the numbers in provide the specifics...