I don't know if the rumors are true or false, just repeating what someone posted here quite some time ago. However, I wasn't talking about general aircraft layout, which can obviously be derived from published specs and pictures, as you stated.
The more sensitive areas would be avionics and weapons. Would Pakistan have to trash a fighter to let someone have a close look? I think some clever PRC engineers could manage to glean some useful info without destroying any of the onboard systems. I also think that since Taiwan operates similar model F-16s that Pakistan has, the PRC had/has a valid interest in knowing some of its capabilities in regards to radar and AA missle performance--I don't think it too much of a stretch that the PRC would have offered something in return that Pakistan could appreciate. However, I guess it is just a rumor--I certainly have not seen concrete proof of it.
As to the US wanting to get a look at a J-10, well, sure the US has more capable aircraft of it's own, but that is beside the point. It never hurts to know as much as possible about possible foes, or as GI-Joe put it "knowing is half the battle"
. But seriously, I think the Pentagon would definitely want to know as much as possible about the the following J-10 (for that matter other PLA aircraft, but since J-10 may go to Pakistan, I will stick to it) systems: radar, RWR, any EW assets onboard, weapons. I am sure they already have a good idea of what threat level the J-10 poses, especially what general aircraft performance concerns (as you implied, knowing an aircraft's configuration and engine performance coupled with computer simulation can make a good estimate of that), and i am equally sure they would still like to know more if they could get a close up peak at one.
But since there are tons of Chinese engineers running around Pakistan and no American (what about European?) engineers, I guess it is more likely that the PRC got a good look at an F-16 than future possiblity of the US getting a good look at a J-10. So it probably wouldn't happen.