I usually buy the best car for the class and have little trouble in challenges.  However, I started the A2 challenges and getting blown out of the water!  I'm talking about worse cars just blowing past me.  I have a feeling that all the AI cars have some level of upgrades with Stuart having all four levels of upgrades.  I can't find a way to see what level the AI cars are but they gotta have the upgrades.  I got the first upgrades and it didn't appear to make any difference.
 
I wouldn't think the dev would force you to dick around until you unlocked all the upgrades. 
 
Also, anyone find the European Tuner yet?

I found a Britsh tuner in Hawaii, on the road from the airport. 

There are 5 tuners in Ibiza, check them out on this map.
 
They are Asian, British, German, Italian, and US Muscle car tuners.

" I found a Britsh tuner in Hawaii, on the road from the airport.  "

" There are 5 tuners in Ibiza, check them out on this map.
 
They are Asian, British, German, Italian, and US Muscle car tuners. "

Ok, I stand corrected, and have therefore corrected my post.