I would like to understand why you would do all this work to make a web part that is viewable by only a few users where all users can see the content, if the content has not been granted unique permissions. What is the requirement?
Also, I have, for SharePoint 2010, lately been seeing lots of permissions issues where customers are setting unique permissions on multiple folders within a library which is causing a lot of problems in the resultant views. (Anecdotal evidence to be true.) Space is cheap. I would recommend creating new libraries (and lists) for each group of content permissions. Heck, new sub-sites, if possible. The trade-off is that you have to manage the permissions, but if the users wants unique permissions on content, probably better if he manages the permissions. Usually doesn't happen that way, but worth a try.