It's something of a misconception that a harder reed is always going to be a harder blow - it all depends on the profile of the mouthpiece lay ( not the tip opening ).
I use Plasticover 5s on my alto and tenor Dukoff D8s - and they work because the the length and curve of the lay is appropriate.
Now, if I were to bung those reeds on my workbench pieces I'd really struggle to get a note out of them...so what I do is use the 5s on them after they're no longer suitable for gigging - by which time they're closer to a 3.
Regards,