sport cups are very good as i have them on my 3 as well. the rears although dont last very long. i had to get my rears replaced at 3500miles, b/c they were already corded. there are several others who have had to replace theirs as well before 4000miles.
if your going to do a lot of tracking, its best to go down to 18's b/c there is more variety of tires to find. then you can do r888's, ra1's, nitto's, or hoosiers
thats why going with 18's gives you more options.I dont know about going with 18's, if you try it let me know how it works out. With the 19's you don't have many options. I think the only tire out that will fit the m6 are the pirelli corsa systems. I have them on my car and their pretty good. And they are very expensive.