Again, the majority of the veterans here know me as the 56k dialup king.
 
I can connect and race successfully to around 350 ms ping.  I can race decently at 400 if only a few cars are there.  I never get timed out due to lag/connection.  It is only when I'm spectating and think I can download a skin or two while not racing.  I have about a 50/50 chance of staying on the server if I do this at 250 ms or better ping.
 
300 ms ping or better servers, I race with 18+ cars on the dialup.  No lag, I ask often.
 
The best server in the list for me is usually Core Racing, which is from 180-210 ms ping usually.
 
It is not true that "if individual cars are disappearing, it is them".  I was connected at a 425+ ping the other night with 3 others.  When the 4th connection came, I started to see cars jumping around and the random individual disappear.  It was definitely my lag, even though not everyone would disappear at once.
 
I of course left when I saw that happening.  Some folks stay on a server even if everyone there says they are lagging.  That is just inconsiderate to others.  If you screw up the race because you lag, then just leave and find a better server.