Hmm, if I'm thinking what I'm thinking...
Set the power target to 100% manually in GPU Tweak, or the highest allowed value. Apply changes, and make sure that "apply overclocking on startup" option is enabled. Dunno if that might help, but I'm thinking your video card is probably accidentally limiting itself. (And have it start on Windows boot. Are you using GPU Tweak II, by the way? If not, change to that version.)
I'll give that a try. I'm using the most recent version of GPU Tweak II, but I can't find any option for "apply overclocking on startup" only one saying "automatically start GPU Tweak on startup" which I've enabled. Is that the one you meant?
First: go to Professional mode, set power limit to 100% or as high as it goes, make a new profile (+ button, type in name) and apply that newly-created profile. If the values were changed, when you press the + button there, the new profile will have the new settings.
Options > Main > GPU Tweak II Startup > Automatically start GPU Tweak II when I log on to Windows
Options > Tuning Settings > Apply settings each time GPU Tweak II starts
Make sure both are checked.
To verify, try running MSI Kombustor's default benchmark (not the furry toruses!), or anything that you were using already that can run right up to the power limits at 100% load.
If there doesn't seem to be any power-limiting, then try using the "default" Gaming profile the app provides for the video card. Do not use the Silent profile, as that limits TDP to 90%.
i5-4590 | Kingston 2x8GB | Asus Strix GTX 970 | Asus Z97-Pro Gamer | WD Black 2TB | Windows 10