Performance boost for iOS rendering with CPU is huge. I am still trying to figure out when it is best to use CPU rendering compared to GPU rendering. I just published an app that is predominantly a scrolling list with some images and with Air2.6, I had to use GPU rendering, CPU rendering would just crawl.
Republishing in Flash CS5.5 with Air2.7 and CPU rendering and the scrolling list flies on my iPhone 3gs.
So you basically have two options if you want to start packaging your apps with Air2.7. You can use ADT that comes with the Air2.7 SDK or if you have CS5.5, you can just replace the AIR2.6 folder contents with the Air2.7 SDK.
Regardless, you first need to download the Air2.7 SDK:
If you have Flash CS5.5, you can replace the files in C:\Program Files (x86)\Adobe\Adobe Flash CS5.5\AIR2.6 with the Air2.7. Just to be safe, I renamed Air2.6 to Air2.6bu and renamed the Air2,7 to Air2.6. It worked!
If you are still CS5, you can follow the exact same directions that are in my tutorial below, just when you are Setting up ADT, make sure you point the path to the new Air2.7 sdk folder.