I have installed a pair of eibach spacer for my rear wheels and i find them excellent! I have yet to test them on the highway as for normal driving up to speed of 100km/h, believe it or not, having the spacing is so much better. I am having ac type 3 18 x 8.5 all around and find the rear too small so decided to install a pair of them for my rear and the tyre shop is only having 15mm available so decided to go with it. sceptical in the first place but when you look at how 'fit' the spacer is attached to your wheel 'hub', you can feel the confidence in it.
bear in mind, with 15mm spacer, you also need wheel nuts that are 15mm longer which they do sell in tyre shops.
will post more review when i tested in highway.