From what you showed us, you are reaching ~1990 MHz (see your 3rd gpuz image).
Have you run 3dmark again?
Also, where are you getting that you're running at ~1500 MHz??????? If it's from 3dmark, thats often wrong....
....rerun 3dmark (let it finish) now (with gpuz) and see what happens...
Until you can show us the clocks dropping, I'm afraid things are running 'normal'. I'll repeat...
Edit: after clicking into your 3dmark scores, I see you changed your system/cpu as well so you're not comparing like things either.
Edit2: I'd bet money t he ~1500 mhz you see 3dm reporting are the base clocks for your card (post a screenshot of GPUz... the first page with all the specs) but that doesn't mean your actually running the test at that speed... again, run 3dm the whole way through with gpuz and check those clockspeeds/post the image here. If you were running at 1500 mhz, your score would be a lot lower.
Your TOTAL score is actually HIGHER on the second 3dm run (because of the cpu/system change). There isn't a problem here at all outside of your gpu score being slightly lower... but that could be driver/overhead. Fromnwhat you shared, you are, clearly, hitting the same clockspeeds.
