APC extension has been superseded by APCu extension. While there is an extension to provide backward compatibility (apcu_bc) MyBB should add support for APCu.
Agreed. This is super-easy to do also - and results in massive gains in performance. I do it sometimes on different websites I build. Love it.
It might be best to target 1.9 or a later version for this. My plan is to replace all of our caching system with Laravel's caching libraries eventually. I had a PR I started on but stopped work to focus on other parts of 1.9:
https://github.com/mybb/mybb/pull/2961