I think the work I have started for
tip 76 would be useful for you. We create lighter-weight versions of acs-subsite that allow for the features of
acs-subsite without necessarily providing any of the
subsite pages, so you have control over eg. what admin pages
are there.
The overhead for subsite creation is primarily parameters
and partly application group. The lightest weight way
to do this would be create a single instance of the user-portfolio-subsite package and mount it at multiple nodes.
What you give up doing that is the ability to have "members" per user subsite and the ability to specify per user parameters via package parameters. It would be lower overhead
at the expense of flexibility.