Both the Ju86 and Wellington VI proved that when you get up really high it is harder to be intercepted but the bomb load has to be trivial and the chance of actually hitting a target minuscule. And there are these things called "clouds" that tend to screw up taking pictures from up there, too.
But the Ju86P proved to be a very important aircraft in the long view. Because the requirement to intercept that aircraft that inspired Stanly Hooker to cobble together a two stage Merlin and when mounted on the P-51 the result was, as Goring said, "We have lost the war."
By the way, the RAF also did "daylight heavy bomber CAS" in Normandy. In some cases, escorted by Mosquitoes, oddly enough.