AIUI the plan is to merge the patchset itself into mainline to ease the maintenance burden, but to keep the changes that don't actually make sense for mainstream server use under a special config/#define flag, PREEMPT_RT_FULL or similar. It will then be up to users/distributors whether to enable it. I've only tried the current patchset casually, but IME, the throughput penalty is there but is barely perceptible for casual use, and the improved responsiveness due to kernel thread preemption and the priority inversion fix do make the system a lot more usable especially under load.