There's a joke in there I wouldn't touch with a 10 foot pole.
I cracked me up though.
There's a lot of variables here. First of all, as you know the laws of physics require we put the effort you add to the cast, not just the weight of the line in or out of the rod tip. All rods have a grain window which as I have said a number of times, in single hand rods is large and forgiving. However, the upper end is likely defined by the likelyhood you will bust your rod if you haul with a good stroke.
The bottom end isLower than many people are capable of doing. A really top notch caster could probably cast a 1 wt. line on a 10 wt. rod. As I said, the grain window for a single hand rod is very large and forgiving.
Then there is the rating stated on the rod vs. what the rod really is. Look at a 6 wt. Sage VT2. It is really a 7.15. There are some, including Sage, that are far more over the stated weight than that. A 6 wt. Guideline Procast XPe is actually an 8.3 There are some makers like Winston that pretty much don't overrate their rods. This is why the whole generic question, "should I overline?" is kind of irrelavent if you don't know what the rod really is. The way the line is constructed makes a difference as well.
The weight of the line that's out of the rod tip is only one variable in a much more complex equation.