I have a 4s. My wifi greyed out when I did the last upgrade (iOS 7.1, sometime around the beginning of April). I tried everything, but nothing worked, so I took it to the Apple store. I was told that the greyed wifi has been known to happen with the 4s and the only thing that I could do was to replace it (for about 150GBP). Since I am nearing the end of my contract with my phone provider, he suggested that I hold out until I can get an upgrade. After reading all the posts, it seems like the problem is with the upgrade rather than the wifi chip failing due to wear and tear.
Since he mentioned that it was a problem with the wifi chip and I kept reading the chip is overheating, I thought I'd try to put it in the freezer as mentioned by several people. I was so scared to break my phone even more, but thought that if it really is an issue with it heating up, then let's see if it works. I couldn't believe it, but it really did work! I feared that malicious people were posting lies, but nope! In good faith, they were trying it and it worked. I've read that this is a temporary fix only, though, so I'll see how long this will last.
I, too, have found that my battery has about half the life span now. I seem to be constantly recharging my phone.
On another note, I really don't see why we are expected to pay more money to fix a problem that Apple has caused. When I'm prompted to upgrade, because it is meant to fix bugs, then I upgrade. Had I had any idea that it would screw up my phone, I would have not upgraded, because up until now, my phone was working without any issues whatsoever. As so many people have had this problem, clearly it is not a coincidence that this is happening after an upgrade? My intention was to hold onto this phone for another year or maybe even two, but if Apple doesn't resolve this, then I will need to get a new phone. Not sure I want to stick with Apple now?