whoa, you have more than 2 performance level? I have only two: 0 ( "2D Clock Frequencies" ) and 1 ( "3D Clock Frequencies" ) besides of 3 "PerformanceMode" of course.
I've overclocked Level0 from "200,100" to "200,401" and I've left Level1 untouched at "550,800" - 100MHz memory in 2D mode is strange since even 2D needs a decent speed of memory copying.
So... couldn't you just overclock your Level1 to level of Level3? I know it's "dirty hack" but hey, you don't have any other options right now.
What's the output of this command:
nvidia-settings -q GPUPowerSource -t