1) Decreasing height will give you more underbodywork downforce. Increasing packers your suspension travel will be limited, so you can decrease height more than when not using packers. But the limited suspension travel will give you less mechanical grip, so you have to test, test and test...
But I think the 1st thing to do in the advanced setup is to find the best spring setup.
2) You have to extract a md file from the track file (dat) using cmagic program. With notepad, edit the md file in the lines "fuel consumption 1 and 2" and then insert it again to the dat file.