There is nothing like new tires to help fix what ails the M Roadster/Coupe. The Dunlopsided tires that come stock are the worst. After doing my darnedest to grind these things down to the treads at track events I decided to get new street tires and dedicated track tires. After a lot of research I decided to go with Yokohama AVS Sports in 235/40 and 255/40 sizes (stock was 225/45 and 245/40). The front tires are a little smaller in diameter then the stock tires, but this appears to be a good thing because the car was just out of the BMW-specified rake spec. With the lower aspect front tires the car now falls right in the middle of the BMW rake-spec and the larger width allowed me to get larger rear tires and yet still maintain the stock front to rear 20mm difference in width. The rears going up one size means the tires no longer have that "too small" for the wheel look and, coupled with the rim protection lip of the AVS Sports, offer much better protection for the rims. The AVS Sports have excellent grip on dry or wet asphalt as well as instant turn in (probably in many ways attributable to the lower ratio in the front).
For track use I opted for the equally impressive Kumho VictoRacers. Not only are these tires the favorite tire in the paddock at most track events, they also are the cheapest R-compound tread out there. The tires are sized as 225/45 and 255/40, but the width of the tires is exactly equal to my street tires. The diameter for the 225/25 is larger. The tires provide amazing amounts of grip on the track (far more then any street tire) and are the biggest performance improvement you can make to a car for track use. I would however caution against it getting R-compound tires too soon! R-compound rubber can mask a driver's bad habits at low speeds and then fling those bad habits back in the driver's face at high speeds. The R-compound also allows more stress to be placed on the car's suspension because the higher levels of grip allow more energy to be transferred to parts like sway bar mounts and shock mounts. So make sure you are ready for the tires in terms of skill and have all your suspension parts checked and strengthened as necessary (like the Dinan sway bar mount gussets) before running with R-compound tires.
My first day with race rubber was a strange one. At first the tires felt like more of a hindrance then they were helpful. I was used to the instant turn in that the lower profile AVS Sports exhibit. The trick (as always) was in the driving. The Kumho's require (and reward) the driver that allows the tires to take a good "set" before turning in. Something as simple as an extra half second of setup for these tires and allows them to take a set where they grip at forces that would have put the AVS Sports into a spin.