Agree with mharvey there. I'm not sure what exactly the gaps are in your inventory data vs SAM's asset inventory data (and I hope you compared it while adding SAM as a WMI node as opposed to an SNMP node). Something worth exploring is Solarwinds Patch Manager. You do need to deploy the Patch Manager's WMI Providers (these are not agents. simply a bunch of files invoked on demand to fulfil WMI queries) but it goes beyind the Windows WMI by extending the WMI namespace. You can get an idea of the high level categories of this inventory information in the demo (see the Compliance Monitoring section)
SolarWinds Patch Manager - Interactive Demo
You can customize the inventory collection and even pull out specific registry keys if you had to.