Up to date current's kernel is 4.11.0rc8 Why are kernel headers 4.8.15?

https://sourceware.org/glibc/wiki/FA...uld_be_used.3F

The kernel-headers package needs to be the same as the version of kernel-headers that glibc was compiled with (that's not strictly true, but I always use it as a rule of thumb). I upgrade the kernel reasonably frequently and I don't want to rebuild glibc every time for no benefit. The private -current currently has Linux 4.11.1, the new gcc toolchain and kernel-headers in sync. Since glibc has been upgraded, it's now compiled against those new kernel headers.

Thanks for that! Is there ever a chance that the spread between versions becomes so great that compiling a kernel becomes problematic?

No. The kernel headers come from having built the kernel. Check out source/k/kernel.SlackBuild.

On a running system (ie server) one upgrades a kernel for security only and leaves the shipped glibc alone (usually) over the lifespan of the (again, usually clean) install.
Unless a found security flaw also mandates the update of the shipped glibc.

Usually, one doesn't run a 7/24 server off -current.

Usually, one switches kernels to obtain missing (new?) functionality vs the shipped vanilla kernel on a desktop/laptop (ie workstation) system.

More recently, "headless" laptops (read decapitated, up-cycled) are ever more popular servers for superior comparative power to performance ratios.

So TL;DR:
just don't worry about glibc, and keep headers in sync to the running kernel, as drMozes kindly advises , they're needed to link anything You build to the running kernel's sys-calls.