I can understand the necessity to get the beta, we are geeks... specially back then in iOS 7 where the main thing was the redesign. now it is a future proof release i get it... but its not worth downgrading your phones battery performance. all the enhancements for iOS 8 like app communication etc are unavailable right now, same with widgets. (theres no apps for that lol) so I don't get the benefit specially when you have a jailbreakable firmware out there.
I've been testing betas since iOS 4 and i've never seen a 20% battery loss in less than 25 minutes, even with iCloud turned off... it might be stable as in "it doesn't at least crash" but behind the scenes the phone runs hot, who knows how the processor is getting hammered or how the network usage is performing in the background...
Yeah, the battery life isn't great at all.