Yeah, "you can't cheat the spec" is the universal contractor's curse. It's like the universe charges a stupid tax for trying to save a few bucks. Always ends up costing more time and money in the end.
Man, that's the worst. Been there with speaker wire, thought I could save a few bucks on a roll of 16-gauge. Ended up with half my speakers sounding tinny and weak. You just can't cheat the spec. That extra cash for the right stuff is basically insurance. Why do we always try to cut that corner?