F35 has received IOC, that alone is a proof. It will be operationally deployed on USS wasp in 2017. This is a fact.
https://sputniknews.com/military/201610251046727095-uss-wasp-japan/
It costs about 15% more than Rafale at the moment and costs are decreasing with every batch.
Agility is not as important for 5th gen than was for 4th gen, a 40G HOBS enabled WVR missile will not differentiate between a 8G and a 9G fighter.
Supercruise was never planned in F35.
It is not underpowered.
The proposed engine upgrade is a part of USAF's effort to develop a next generation engine for powering future aircraft. An upgraded F35 on the same engine is also a possibility, but it in no ways means that F35 is underpowered. If you claim otherwise, prove it.
Anyways, such engine upgrades are normal part of an aircraft lifecycle. Rafale has also gotten its engine upgraded multiple times, in fact if there is one plane with not ready engine, its PAK FA. It is still waiting for its
izdeliye 30 engines, and reportedly IAF has refused to accept the Item 117 engines because they are simply upgraded versions of engines we have in Sukhois. F35's engine was built from scratch. It much more recent than PAK FA's current engine.