Assuming that U.S. government agencies share information with each other is a big assumption in most cases. I highly doubt DHS knows who does and doesn't have a security clearance when they do their review, although working for another federal agency might boost your credibility some. I suspect they're just reviewing databases with criminal records, maybe Lexis-Nexis for some cases, but that would even be a stretch. I still took a month to get conditionally approved, which was about par for the course around the time I submitted my application. For my interview, other than the obligatory questions they have to ask, we just talked shop.