If you want a high end motor then no need to look further than the 19series lehners.The 1940 is a fraction smaller than an XL and the 1950 is slightly bigger.They use segmented magnets which makes them more efficient under partial load(where they spend the vast majority of their time in a racing application).
As for which turn,as a general rule you want to keep your max rpms to between 35000 and 40000.Go here
http://www.bk-electronics.com/ to see the Kv values(rpm per volt) and specs for lehner and wanderer motors.
eg. if you run 12 cells or 4s lipo,the 1940/6 would be a good motor.(14.4x2613=37600 rpm)