Well, for directory lookups CP/M 3 with HASH buffers would work better. I guess if you have enough track buffers then you should essentially have the directories in memory most of the time.
Regarding the overhead, keep in mind that you are adding that ON TOP of whatever I/O needs to be done, and it is not insignificant even on a Z80. CP/M 3 will do bulk I/O (with multisector count, if the application uses it) directly between disk and the user memory, so that could be faster than using a track buffer and copying. The raw math doesn't always look as good in real life.
I also wonder, if you are caching multiple tracks, what the overhead will be when you sync to disk. lots of stepping to do in that case, you may need some sort of "elevator" algorithm to optimize it.
You've got a big project ahead of you, best of luck. Lots of interesting problems to solve.
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