I'd have given Raven Guard re-rolling reserves, Outflank, and made Winged Deliverance the ability to assault on the turn you arrive from Reserves as long as you didn't Deep Strike.
I think it's ridiculous that they're 'the sneaky Marines', because Marines aren't sneaky - their Scouts are, but Power Armour makes a noise like an idling V8 and that's just standing still. You might be able to sneak up on a half-deaf Ork sleeping off a hangover, but not much else.
I think the idea that Raven Guard are 'stealthy' is a bit of a misconception. You're right, you can't sneak up on someone wearing power armour.
I see Raven Guard more as masters of guerrilla tactics, ambushes, hit and run attacks etc. more than straight up stealth.
Plus Corax and I believe some others have the ability to make people not notice them even in plain sight. I don't know if that's a skill that has survived 10,000 years.