Gallon of milk...
I see your point that with side pressure you may be able to get considerable more pressure and that is exactly how I fight the fish. The experiment I was mainly thinking of was attaching your tippet to a gallon of milk and trying to lift it. I think a gallon ways about 7 pounds and even with a stiff 10wt GLX I was not able to lift the container and was putting a dangerous bend in the tip. But I guess if I turned the rod sideways the way I do with a fish I could probably drag that or a heavier opponent down the street. I guess if I pointed the rod straight at the fish and took all the bend out of the rod I could exert as much pressure as I wanted but that wouldn't be very much fun.