I figured a good way out to make a good amount of Skillpoints.
i created a bucketlist-challenge at the GoldMine called "XP" - just add me as a friend on XBL and you should see the bucket list.
You just have to drive along the lonely highway without other traffic and collect your ultimate-speed-skillpoints
you can add Wreckage-Skills when you crash some poles near the street
just go up and down nearly to the time limit and then finish the race.
best is, when you have an "HE"-Car with Skill- or Speedpoint-Boost
i managed to get about 150k Skillpoints per direction from start to finish