Well, I jailbreaked my iPhone 5 just so I wouldn't be plagued by this update. I guess I can't avoid it now. My jailbreak (Pangu) isn't stable so I have to do a DFU restore which will update automatically to iOS 8. I dunno how people who jailbreak can put up with that the instability.
Once I update, I will report back and let everyone know how much RAM is sucked up by iOS 8.
Well, my pangu jailbreak has always been been stable on my 5S. Truth.
iPhone 5S Gold ( iOS 7.1.2 )