Well.. to be honest, you are right in some way. Z1 actually is a new phone, I own one too, bought it in pre-sell for inflated price. I know it has issues and for this price I am not very happy about them.
But try to look at it on a little different angle, just for a second. People kept complaining about the Xperia Z. It was a great smartphone, really was. But time shown that the camera is not that good, the waterproof glue on the back cover didn't make it when the handset got too much hot, the speaker placed on the corner was a mistake because ppl unintentionally covered it with hands, and the flips were not as perfect as they were suppose to be. So now we have Z1, all above heard and fixed.
But people still was complaining, when, when when the Honami will be released. So Sony rushed the phone to make this wish come true. But any rush in this matter have a cost. So this is it - we have the phone much earlier than it takes to make it perfect, and now we hate Sony for the very same thing we asked.
I remember the same thing about Xperia S, the flagship too. Once released it had many problems, some updates were delayed, other rushed and hated the very same way. But today it is one of the most stable and perfect phones, after so much time it still amaze. I am sure Sony hear us here. I am sorry about your problem, I really do, so I volunteered to help the best way I can. I love the Z1 regardless of all those issues, I really can live with them. I also updated today to the firmware that causes you problems, there is completely nothing wrong after this, just battery don't drain that fast as before, so I believe a simple SUS repair can really help you with your problem.
I really wish you solve this, because heat could damage your Z1 before it gets to the service. So this is what I would do with my own phone - backup, try the SUS, and if that fails then just roll back with Flashtool to the previous firmware. I faced similar problems myself, so I can understand you mate.
And I can assure you, Sony listens.
