The best drop I've seen for all naturally aspirated 6's are with Mazdaspeed hatch springs. These springs will work on the sedan versions as well. Don't get the Mazdaspeed sedan springs because those will sag in the rear.
The Mazdaspeed springs provide a modest even drop, are linear, and have a good spring rate increase to match the drop for good handling. My next mod will be to swap out these Tanabes for the Mazdaspeeds. I'm thinking a very good suspension setup for my 6 would be Mazdaspeeds with Koni yellows and a Racing Beat rear sway.
Regular Eibach ProKits tend to sag in the rear for the 6's. That and they're progressive and their spring rates are not exactly tuned for the drop they offer. Like the Tanabes, I consider them more for show than go.
Oddly, Mazda developed the Mazdaspeeds with Eibach and the Mazdaspeeds turned out way better that what Eibach came up with. Thank goodness for that.
As for your 18's, you'd have to tell us the width of the wheel, offset, and tire size so that we can advise on fitment. Have too low of an offset and you might need to roll your fender.
