Got my class B last night, have 13 cars I think. I love the stunt cars, use them for everything as I just can't get used to the boost limits of the speed cars.
In those takedown missions I kinda go overboard too, they are so easy when you get on a freeway or something and I end up with 4 or 5 times the number of takedowns needed. There was one though that was hard out of nowhere, I had to drive carefully to get past 6 and ended up with 7.
All the stunt runs are still extremely simple at class B though, usually get the goal on my first run with 1-2 minutes to spare. The races are pretty hit and miss, I failed a few lately as the difficulty has ramped up.