Battery impact should be close to zero. Xposed does not require any services or additional wakelocks. Also it does no polling (frequent checks for a certain state). There is a small initialization phase, afterwards it gets only called when one of the hooked methods is about to be executed. So it does not prevent deep sleep because it gets only triggered by apps/services running anyway.
This of course depends on how the modules are implemented. The red clock example hooks a specialized method that is only called when the clock text is updated (once a minute). If it instead hooked a very generic method like setText, it would be called more often and it would have to execute more new code to find out if it has to do something.
The same applies to performance. If the developer is specific with the method hooks and the ratio of original to additional code is sensible, the impact will be low.
These are mainly thoughts as I could not test battery impact over a longer period yet (too much develop and restarts etc). But I did not feel a performance impact from the clock example, so the framework should be fine.
Tired of updating your mod for every new ROM release?
You want give users the possibility to combine different mods without creating tons of different files for all possible combinations?
Then have a look at my Xposed framework
- modifications without APK changes!