Well, there is no point on trying to reduce disk IO load (mod_cache is effective only on static data, and that for local data means disk IO load) by caching on disk. Also, I suggest you try to find the real bottleneck, because I'm not convinced you did.
Those are the reasons you may have too much load on some websites:
- disk IO is full and Apache is constantly waiting for read or write access on static data
- disk IO is full and PHP is constantly waiting for read or write access on static or dynamic data (to create images thumbnails or use temporary files, for example)
- CPU is full and Apache is constantly waiting for access to do something internally
- CPU is full and PHP is constantly waiting for access to do something internally (creating opcode, for example)
- CPU is full and PHP is constantly waiting for access to run opcode
Each (and more, if you use any other feature/interpreter/database etc) of those possibilities have a completely different solution, so start auditing your server and I'll be able to help by reading the results.
The easiest solution, that should work with almost everything? Buy massive amount of memory, that costs nothing, and cache everything (from static data with mod_cache, to PHP opcode with opcode cachers to MySQL tables, indexes and queries, etc) into it with reasonable parameters.