nope, it wont increase (or decrease) the stiffness of a rod.
What it will do is make that rod act weaker than its marked. Ie, load it with less line in the air with a like line. Keeping in mind there's a crap load of lines to pick from, so much so its gone to a form of confusion. And its visa versa, throw a 4wt line on and it's going to take a pile of line to get that rod loaded. If you go too far, the rod will collapse if you have too much line in the air. Its fine for a guy who likes to shoot (myself) vs cast. Instead of trying to keep 90' of line in the air, you get 30' up and shoot the rest. It will also throw heavier or more wind resistant flies easier due to the added line weight. Generally speaking a poor casting rod is usually due to a poor line match more than the rod itself.
A rods number (or designated rod weight) correlates to a given range of line weights in grains that it takes to load the rod....I forget the standard distance but 30' is screaming in my ears. By going up with a similar line its going to take less line in the air to load it, by going lower it takes more (with the same company and model line). There's so many different line designs on the market and companies for that matter that swapping any one thing can completely change how the rod reacts. On faster rods I generally as a rule go up 1 line weight for a wf floating. You may find that a DT line casts better than a wf line or visa versa. Due to the added weight in the air. Once you get past the length of your WF line of choice, the DT is going to be adding more line again.
So if you're casting short streams with a fast rod going up a line weight or even two, or buying a line with a short front taper will get the rod loaded faster than a longer front end line. Or if you're doing no close up work, say swinging for salmon/steelies, you may find you want a longer bellied line to load farther out. I prefer to shoot line, not cast it all day long. More air time equals less fish time.
Think say a pike/muskie taper vs a traditional wf line. In a similar line weight say 8wt, a SA Muskie/pike taper will load faster and throw bigger more wind resistant flies with ease compared to a similar 8wt sa mastery wf.
The line cannot change the rod. the rod is what the rod is.
Another tip, mark the running line end of your fly lines...after awhile you're going to have a pile of them if you don't already. I use black bands to signify weight class's, and red bands to signify sink rates. Wide bands equal 5, narrow bands equal 1. It takes the guess work out!
