still have a diffuser like that for sale. If you're interested, pm me.
I don't know if it's just between the ears or not, but I have the idea that I feel the difference with and without diffuser. It gets a little more stable on high speed corners.
I've got one already, I haven't had muh chance to test mine but I made rear suspension changes at the same time as fitting my diffuser, so was expecting more stability from the suspension anyway.