Obviously, you would break your neck if you tried descending the real thing at high speed, but if the road were straight down with no gravel, potholes or wandering sheep (!) to cause problems then I'm sure that you could actually exceed 105 kmph/65 mph!
I once hit 88 kmph/55 mph freewheeling down the A58 from Blackstone Edge towards Littleborough and that stretch of road only descends at about 7%.
If you have the trainer setting on 100% difficulty. 16% will be plenty.
There are very few inclines within Zwift that exceed 16% and if they do it's only for a few yards
I wouldn't be complaining about it not feeling hard enough, just that 16% is not a realistic simulation of Hardknott! I get by ok tackling my local 16% climbs (slowly!) but suffer badly on 25+%. 16% wouldn't feel especially challenging compared to the real monster, which I have thus far put off trying to tackle.
Overall, I'm sure that I will be perfectly happy with 16%! I took a quick look at the
Decathlon Van Rysel but dismissed that as an option when I saw that it could only simulate 6% - I have sprinted up local 8-10% ramps on a singlespeed bike so 6% is not much good for serious use!