Hi. I think that this is something that can be fixed in the JFusion code. The extension can add it's own paths for loading models. To avoid conflicts it should use the prefix option:
JModelLegacy::addIncludePath('extension/models/path', 'JFusionModel');
Then the extension can invoke the model like that:
JModelLegacy::getInstance('usergroups', 'JFusionModel');
If that does not work we can try to remove those lines from our system plugin however this is something that will take some time since it requires testing. We need to be sure that K2 will not break in any of the Joomla! supported versions ( 1.5, 2.5 and 3.3 ).