After wayyyyyyyyyyyy too much formatting/reinstalling. Uninstalling storage drivers, etc etc. I've finally figured out why it was dropping me to 1.5 Gbps -- seems like that the RST drivers when installed on a laptop it enables a feature to 'power save' -- Link Power Management (LPM)
The fix was to change in the registry
HKLM\SYSTEM\CurrentControlSet\services\iaStorV\Par ameters\Port0 (corrosponding port that you are having the problem with. Change the LPM DWORD value to 0, and it'll disable the LPM and put it back into 3.0 Gbps intead of 1.5 Gbps.
Posted for posterity's sake in case anyone ever have this same issue.
Caused me over 24 hours of grief.