There can be inherited results after you connect new hardware. You probably have done so already but worth noting to have all the latest drivers installed. Then clear out the previous WEI scores the following way:
Go to Start Type
%SYSTEMROOT%\Performance\WinSat\DataStore
in the search area Open the data store folder
Delete all .xml files
Close and go rerun the rating.
Info for other methods are located at Microsoft.com
Remeber that Vista's max rating will be 5.9. Any comparision as far as WEI goes past that was measured with Windows 7 (not sure if 8 has WEI).