Once China has produced J-20, the stealth tech won't be new to them. Same is the case for low-noise producing helicopters and according to US defence analyst, same technology is also available in civil helicopters. From the wreckage, when China would be learning US tech, it wouldn't mean China would be learning ABC of these technologies.
But I must say there would be a lot to learn for Pakistan and even for China form that. The Glass Windows had a special coating on them which can be the same coating used on F-22s. Learning about US composite materials would be another great aspect for both of them.
Another important aspect of this learning would be to understand what US was capable of and hence US's stealth technology would be more predictable than before.
This will also give China some queues to build better radars which could see the best known stealth technology America has at present. So its going to pinch America in more than one area because of OBL who when was alive was a trouble to them, now when he is head, he is of more trouble than he was alive.