The "New Z4M" is called a Z4 sDrive35iS.
I had an E89 Z4 before my 350Z and got abit bored of it, loved the looks and still do but it felt to much like a normal car to drive, it didnt have that sport car fun factor, like you said you might aswell have a 1/3 series.
Sounds like the E89/E90 on the motorway also so i suspect its the run flats, my girlfriend didnt feel safe in the car and it jumped around alot, the 350Z is just planted and feels alot safer in the wet.