I've been using Dashboard Feeds plugin for awhile now, but I've noticed that for some time, it hasn't worked. Older blogs have the custom feeds I put in, but newer blogs don't have them (they display the WP garbage feeds). It seems like they are impossible to get rid of. I even tried this plugin, in addition to yours and it still didn't work. http://wordpress.org/extend/plugins/wpmu-simple-dashboard/
The interesting thing about that plugin is that it successfully renamed the dashboard widgets as I set it to, but it failed to overide those persistent WP feeds. Looked kind of funny seeing my news feed titles on the widget, but Ma.tts latest ranting shown in the feed.
Why is this so hard to get control of? The last thing I want to do is a hardcode edit (with subsequent upgrade issues). I'm just about ready to remove the widgets except that it is a useful feature, it just seems hard to get control of.