I searched the IBO web site for a (incremental) list of enhancements
and bug fixes and was unable to find one.

An eMail to Jason resulted in an automated "message read" response as
suggested by Jason. However, so far, no information.

Do I have a blind spot about where this information can be found or is
it not available?

I am currently running 4.3Aa and want to evaluate the cost/benefits of
upgrading (most of my clients do not use Firebird servers).

Thanks,
Clive.