Depends on the settings you choose for the kernel, in some kernels you will get a better battery life and better app performance, and some kernels are pure performance and some even focus on battery life, most though provide both features allowing the user to choose what to use and not use, so you can choose settings on custom kernels to reflect what you prefer.
For example, in my case, I use ElementalX and use its governor, set the max cpu to 1.7ghz and max gpu to 450mhz, then I use an app called Faux123 kernel enhancement pro (there are free alternatives out there) and make individual profiles for apps, so the apps that do not need heavy cpu or gpu, I can tell the kernel to throttle the cpu and/or the gpu to save battery, while I will set a 3d game for example, to max out (slightly overclock in my case) the GPU and CPU getting smoother performance. Of course, battery savings is app and usage dependent and YMMV.
ElementalX uses something called Aroma which lets you select various settings just before flashing it, it adds other functionality, like loadable module support, exFAT support. various governors, max read-ahead settings, and so on, they are outlined quite well in
this thread.
Do not feel as though you must use a custom kernel if you have no need for any of the features and are happy with stock kernel performance, the stock kernel is a good kernel, it just has limitations.
And yes, gravity box will let you do as you asked, it includes pie controls of its own as well as a way to make the nav bar go away, in fact it is how I use it. If you find its pie controls too limiting, you can use LMT. Personally, I think the built-in GB pie controls are fine.
...keep in mind though GB is geared towards stock roms, it is not recommended nor supported to use GB with a custom ROM.