iOS 8.1.1 is general bug fixes and improvements (for all devices), INCLUDING performance improvements to iPhone 4S and iPad 2. The latter is the update's major feature, so the way they describe and focus on it, makes it sound like it is for those two devices only. But it is not - it brings general fixes to all devices (I googled it).
If an update is really just meant for a certain device, they will say so explicitly and it will be labelled as such properly (think about iOS 7.0.1 and 7.0.5, which were meant for iPhone 5S and 5C only). Normally, other users can still download such an update, but it will definitely say "for xxx devices only"...which iOS 8.1.1 clearly doesn't do.
So what I'm trying to say is, this update is for all users, so go ahead and download it. The general feedback from users worldwide is positive. The other advantage of 8.1.1 for iPad users - it gives back some storage space, meaning the OS takes up less space. I can confirm, was shocked to see 8.1 taking up a full 1Gb more on my iPad Air, but got 400Mb back after updating to 8.1.1
Updated both my iPhone and iPad to 8.1.1 today so still too early to properly comment on battery life and general performance, but as mentioned, general feedback from all across the globe is positive so I don't foresee any problems. But if 8.1 was already working well for you and not making any trouble, you probably won't notice a difference.