Not many DBs can run with WRs (which is why they play defense), so yes I do believe that with your skeleton drill and mixing up the routes the fly route would be completed well over 50% of the time. DBs are typically slower than WRs and they are always hedging that the WR is not running a deep route to keep from turning and running only to have the receiver break off the route. I don't know if you've ever played DB, but when someone is coming at you full speed you have about a one second window to decide to turn and run to defend the fly. The receiver already has a full head of steam while you're in a soft back pedal. If the receiver is faster than you, or if you hesitate for a second, there's a 5 yard gap between you and the receiver that will never be made up. Hence, they came up with Cover 2's to give DBs some help over the top.
I don't know. I guess I'm just not good enough yet, because there's no way that I could throw for 500 yard/gm with Ginn, Bush and Calvin Johnson on the same team. I keep misreading the defense and getting picked by the safety. I've also seen the DB outrun my WR to overthrown balls on fly routes and make the pick more than a few times.