Removing uneccessary software will not improve CPU resource availability at all. The code is unused, so it uses no CPU resource. It may provide additional memory if the software being removed is part of the resident kernel. If the code being removed is normally a module, then there is no memory savings - if the module is not loaded, it uses no memory (or CPU, obviously).
There are changes that can be made to reduce latency as you've made with preempt, but that reduces
available CPU cycles in exchange for faster access to the CPU. Asterisk does not require that as far as I have been able to determine.
There should be no tuning necessary (other than running the most recent 2.6 kernel possible). Asterisk has no requirement for user specified kernel tuning. If you are having a problem with dropped calls or other issues, I suggest that you post a question in a new thread on that specific issue.
There is also an Asterisk Forum
that you may be able to find help in.