Apologies if this would be better placed in the Technical section, but I'm posting here for now!

OK, so I've seen a couple of blob-eyes with the latter Impreza diffusers fitted recently and I'm becoming more and more tempted to put one on myself.

I'm curious though: for a car of relatively standard specification, does the rear diffuser make the slightest bit of difference, or is it a purely cosmetic addition? Subaru fitted them as standard on the Hawkeye, so I assume they are of some benefit to the car rather than just an extra bit of plastic to lug around?

Thoughts on this anyone?

 

Rear diffuser without full flat floor is pointless and just cosmetic addition.On some cars(Rally cars,Race cars etc.) is full flat floor which create positive downforce(something like suction effect),best way(for relative lower price,than full flat floor) how get bigger downforce is APR rear spoiler(fully adjustable and which is tested in aero tunnel).I'm not sure but these diffuser(HT Autos) are not tested in aero tunnel,if you are looking for aero tested and mostly used out are APR(which are tested in aero tunnel)
Full benefit from just diffuser,is not possible due you don't have as above i say Full flat floor....
A diffuser takes the airflow from under the car and channels it out into the low pressure area behind the car. The shape and design of the diffuser will determine how much (if any) downforce is produced. A full flat underbody will work the best with a diffuser, like the picture of the Enzo .
On a car without a flat underbody, the bottom of the body panels and rear bumper is usually lower than the bottom of the actual chassis of the car. A diffuser will help keep the airflow smoother when flowing into the area behind the car.