I don't use a level line, but I do use a level leader for deep nymphing and find that it helps a lot. From the butt, I attach 2x or 3x and run about 7 ft of that. Then a blood knot attaches about a 2 ft piece of 4x or 5x to the point fly. If I have a dropper, it goes off the blood knot tag end on the 2/3x.
Harder to cast, you have to open up the stroke and do more of a "lob".
But the fly sinks better because it cuts water better. And more importantly, you can keep a tighter line when high sticking. The reason is the more constant diameter, and thus more constant drag factor throughout the leader.