Which is kind a what I'm doing...

Lets say I have two modules A and B. Both these modules are part of the lower layer described in my orig. post. Lets say Module A goes and fetches some user information from an RDMBS and module B goes and fetches some information from an RDBMS. Both modules are abstracted to their respective domains (classes), both fetch different bits of non related information.

For maintainability its easier to have both modules A and B create their own db handle fetch the data and return it to the upper layer... for Performance it would be better for the upper layer to pass a pre-created db handle (object) down to the bottom layer - which makes modules A and B not stand alone (or self sufficient). This need not apply exclusively to DB handles (I chose this example because of the overhead in creating/destroying DB handles), but any "helper" module that may be used more than once by the lower layer.

What I’m hearing thus far, is maintainability matters, of course it does, but so does performance.. hence the trade off.. ;-)