How much more storage than iOS 7 does iOS 8 ACTUALLY use up?
The Ars Technica iOS 8 Thoroughly Reviewed article pegs it at 600MB difference on the iPhone 5.


DEVICE  SPACE AVAILABLE    (IOS 7.1.2)  SPACE AVAILABLE (IOS 8.0 GM)
32GB    iPhone 4S (AT&T)    27.4GB      26.6GB
32GB    iPhone 5 (VZW)      27.3GB      26.7GB
32GB    iPhone 5C (AT&T)    27.2GB      26.5GB
64GB    iPhone 5S (AT&T)    56.0GB      55.1GB