I would say bettery is about the same of iOS 26, but mine is quite cooked (78% battery health) and I have to replace it.
Performance is for sure better. Not as much as I expected (I expected to notice it everyday), and I can't give you a number, but I remember that I noticed stuff being faster the first couple of days that I installed the beta, but now I got used to it
I run iOS 18 and have no problems with my battery, it is at 73%. iPhone 14 Pro. Don't think I'm going to install 27 either.
I reviewed the battery test results for iOS versions 26 (26.2, 26.3, 26.4, 26.6) and 27. The comparison covered the iPhone 14 series and newer models. The batteries in each model retain their charge 10–15% less effectively on iOS 27.