Probably. This has always happened and will always happen. It's no different than the Soviets stealing US nuclear tech 60 years prior.

Stealing the plans is the easy part. The challenge is building an integrated military system top to bottom, from engines to electronics. If it were so easy to keep up by stealing, China would have a dozen aircraft carriers by now.

The J-20 seems to being coming along nicely, however.

Any information about how it's actually performing?

It's a mixed bag thus far.

China has successfully flown three pre-serial production prototypes that met spec. It's been spotted with ELINT and weapons hard points. Full production is slated for 2018.

However, they've been having severe difficulties with engine development. Most of the demonstration flights were done using outdated engines (usually the WS-10.) Once the production engine is functional (the WS-15), it should be able to lay claim to being a true Fifth Generation fighter and F-22 competitor.

If they can't get the WS-15 working, they'll probably have to buy Russian Su-35's NPO Saturn 117S engines used in the Sukhoi Su-35, which they've been negotiating, but it would make the J-20 more of a 4++ Generation fighter.

The front stealth shape is nearly identical to the F-22 Raptor, however it uses canards to give it some aerial superior advantages at a high angle of attack. But this comes with a tradeoff of some stealth, especially on profile as opposed to head-on, and sacrifices aerodynamic stability making it heavily reliant on its fly-by-wire system.

To a group of suitably trained engineers operating in the confined solution space of high manoeuvrability turbojet aircraft with human pilots, could enough information be gleaned from what is freely available knowledge and understanding? Is theft necessary?

If you can steal the plans for an F-35, you won't need to spend as much money to develop your own. You can just build one and test it to see how it compares to anything your own engineers came up with along with the American-made F-35 test results you stole too. Then you can spend that money on trying to fix anything that might be wrong with the American design.

Sure, with enough time and money, your guys might have been able to come up with a better design but if you can steal at least something to build off of, why not? It's cheaper to pay some kid hackers to steal it than design it.

When I see the photos of the stolen Chinese version of the F35 it always reminds me of this:

https://en.wikipedia.org/wiki/Siberian_pipeline_sabotage

Worth remembering when people talk about how the US military budget is bigger than everyone else's put together. It doesn't mean quite as much when everyone else is just waiting for you to pour trillions into R&D then stealing the result.

If stealing information were the key, Russia would have the most powerful military on earth, with a combination of their espionage of US military technology over 60 years and their very substantial defense budget.

It's drastically more difficult than that.

The US military also doesn't spend trillions on R&D - R&D is the light end of their budget ($80 billion per year for R&D, testing, evaluation). Operations, maintenance, and personnel are the principle costs of the US military budget.

You're talking about China (or Russia etc) stealing a fraction of that $80 billion total. The amount of that theft they can then actually put to use, is a further reduced fraction.

And China is mostly stealing from Russia at this point (well, buying military hardware from them to copy).