Hehe you didn't really have to share THAT donor since you seem to have used a lot of different ones, which is all good. But it's good you are aware of us wanting to see donors.
Overall I think it's your selections that lower the execution's level. They are either too sharp or jaggy. The mirror is very choppy/square, edges of the diffuser show red, front arch especially is sharp.
What you can do is go over the edges with a low percentage blur tool + ALWAYS use the pen tool for selections!
The vent in the rear arch isn't that good either but the rest of the bodykit looks put on better.