Is there a CPU overhead in case we increase the number of pages in a database buffer?
Increasing the buffers has no effect on CPU. The big problem could be z/OS paging. If that doesn't happen, then you should not see any noticeable change in CPU.
CA-IDMS uses a hashing algorithm to find DB pages, so the number of pages doesn't matter. It also builds bit lists, which depend on the number of pages. But the access of the bit lists should only add a minor amount of CPU, most likely not detectable.
The big advantage of large buffers is the re-access of a DB page that's still in the buffer pool. The bigger the buffer pool, the longer the page stays in the buffer. If that page is accessed many times, then the I/O is reduced. If it's only accessed once or twice, then a large buffer pool has no advantage over a small buffer pool.
More information about the sizing of database buffers can be found in the manual:
“Database Administration Guide”, chapter “Buffer Management”
Release: IDADSO00100-18.5-ADS-for CA-IDMS