You don't downsample to 2560x1440, you downsample from 2560x1440, but yes, that would show you the performance you'd get.
Oh, okay.
Still don't really understand downsampling, that well, but as long as I'd be able to test the expected performance.
Downsampling makes the GPU create images at a target resolution. In this case 2560x1440. Afterwards, the images is resized to fit your actual resolution.