MashaAllah very nice attempt and surely a wise way to spend the money. There is an odd cut-off between the black and white when viewing the 'integrated diffuser' against the mud-flaps. That can easily be fixed by removing the mud-flaps. The next thing that you will face as an issue will be the empty space in the rear wheel well. For this i found a little handy trick. You can create a plastic inner shield from the old mud-flaps by cutting them very carefully using a heated knife. Then sand the edges and smooth them out. Your wheels are of correct off-set. The mud-flaps don't belong here in my opinion. However, if you are happy with it that is all that matters. Kind Regards and MashaAllah again
Edit: I actually had an inner-shield from my 2008 Si for the DIY. So i don't know how well you would be able to replicate. That is obviously dependent upon if you want to do this at all.