NikeX, to Yorkie's point, systems integration / fusion is a major issue for all aircraft manufacturers, BUT, I wouldn't overstate the point that the US has "mastered" this. The much touted JSF program has been through 3 major re-structures in the the past few years and the program is still struggling to correct latency issues with the HMS, ongoing issues with stabilising the software and fully integrating all the elements into a fully combat capable system and the inherent complexity of its logistics management system, on time and (no where near) on budget ... even the new incoming USAF Major General Christopher Bogdan, deputy program executive officer, has quite publicly stated this as recently as a few days ago. I can't insert the link, but search Business Week "Air-force-official-slams-lockheed-martin-on-f-35-program" published 17-Sept-2012.
You're right to point out that the engines and therefore the inability of current Chinese fighters to have access to high thrust engines that allow super-cruise remains a key weakness, but one that one could reasonably expect in the next 2-5 years time will be resolved. The Russian's have started testing the PAK-FA with engines are not indicative of the final build, on the expectation that higher trust, production ready examples will be ready in time. The measured and deliberate steps as well as the massive resources that Chinese is now devoting to correcting this key strategic weakness suggests that they'll get there in a similar time frame and in-time for production ready examples in 2017-18.
IMHO, the key is not whether or not the Chinese stealth fighters (J-20 and/or J-31) are 100% as good as or better than the US planes across all aspects, rather, 'good enough' for their introduction to effectively counter a key strategic advantage that the US once enjoyed over virtually all current and potential rivals. Reduced signature aircraft blunt the effectiveness of medium range AAM, as the acquisition window for the radar in the AAM to acquire the target is reduced in the same way a planes x-band radar can't detect or effectively lock-onto. They greatly complicate deployment of counter-air, key ISR and logistics (tanker / transport) assets movement.
It then becomes a game of "political will" - if I was to deploy my forces and be assured of an 100% success rate against mission objectives with minimal casualties and loss of assets... there's a higher likelihood that you'd pursue this option, relatively to a scenario that sees you with a significantly lower % probability of achieving all your stated mission objectives and/or all the while incurring a significant or even unacceptable loss of personnel and/or material to achieve your objectives...
