Not sure about stormy's Since im just getting back into SH after almost 2 years
As for getting into the ring of destroyers, Get directly INFRONT of them (about 5km) and go down to scope depth. when they get inside of say 2km, go silent, drop you're scope and let them Pass.
I have
Never seen destroyers go active UNLESS THEY DETECT YOU. which means if you stay stealth early in the early war they can pass within a few hundred feet of you and never notice- if you shut your motors off completely. They also cant hear Behind them very well. After they pass a good bit raise scope and maneuver into position and start taking down ships. Try and be in the center of the convoy when you do this as all the merchants kinda cover your own motor noises.
I'm in late 1940 right now with the VIIC, And this works quite well for me. also once the 'deed' is done dont linger to watch the fires :P dive down in silent running and get out of there.. drop one decoy too for good measure. Vanilla DDs always go for them.