I'm the type to put the game on the hard difficulty rather than easy because i find it more rewarding. i even play forza with the clutch mapped to the x button. start slow and work on the shifting technique, hitting apexes, etc. the speed will come later. if you're mis shifting its because you're too worried about the corners (coming in too fast) and losing track of everything else. using the proper technique (palm on the left side of the shifter with your thumb down) when going from 3rd to 4th also helps so you don't do a 3->2 money shift.
https://www.evo.co.uk/porsche/911/13...vs-pdk-gearbox
^ great article by Evo magazine. comparing the 911 pdk vs manual. the manual was faster but that was due to the better tire setup.