I finally got gold in the Samba bus race. Haven't tried the Elise one yet.
Here's a breakdown of how I did it, as detailed as I can remember for those who are having trouble:
Shift points are:
3rd - 32-33mph
4th - 51-52mph
This is important because if you miss a shift or short shift, you'll fall behind and miss your chance to pass.
Start Lap 1 - Hit 3rd and then 4th gear, slide in behind the blue van asap.
First corner - brake down to 45-48mph, shift into 3rd while braking, make the right. You should be right up the first guy's trumpet.
Into Chicago - the first guy will brake hard. Don't. Go to his left, pass him, the next guy, then swerve right, cut inside and pass the third guy before the tires in Chicago. Stay in 3rd the whole time. If you've done it right, you ease in behind the yellow van on the exit.
Stay on the yellow van for a bit, get right on his butt for the straight leading up to Hammerhead. He may duck left. If he does, slide past him and in behind the gray van. If not, try to slingshot past him in the straight using the slipstream.
Hammerhead - Late brake, shift down to third on the left hander, be on the right side. You should go into this corner 2 up with the gray van. Shift down to 2nd just before the right hander, punch it. If you managed the slipstream right on the straight leading up to the corner, you'll be behind a blue and a turquoise van now. They'll stay 2 up all the way through Bentley.
Bentley - Can't really pass anyone here, but get as close as you can to either the blue or turquoise van.
Follow Through - You should be able to get into the blue van's slip stream and shoot past him and the turquoise van.
Bachrach - You can late brake into this one but be careful. It's a downshift into third but you almost don't have to do that till you get around it. Just watch you don't slide too much. If you missed the slipstream on the follow through, you can make up some ground here. I can usually pass between 1-3 vans here depending on who's there.
Gambon - Tough to pass, but downshift into 2nd if you fall below 35 and punch it. You won't win across the line, but neck and neck you'll have the better line down the straight if you're on the left.
Lap 2: Things get a little frenzied here. Just stay cool and keep on your line. By now you should be between 5th and 7th place.
Follow the same directions for where to pass as the first lap. If you play it right, you'll be in second place by Hammerhead. That leaves you plenty of time to catch up and draft the first place van. I shot by him at the midpoint of the follow through, but YMMV.
-Where to cut the corners-
This challenge is exceptionally cheesy with where the corners are and where you can and can't cut them. Some places are very strict and some don't mind at all.
Chicago - You can cut a cone-width on the left of the left hand lead-in. Before the right hander, you can go as wide right as you want, as long as you make it back in before the tires. Chicago itself is pretty strict.
Hammerhead - Don't go too wide left or right on the straight up to the corner. Too far left and you can't get back in to make the corner, and too far right and you'll hit a distance marker.
As you enter the corner, you can cut the left hander a bit wide, as long as you make it on the left of the flat marker in the ground at the apex. Past here, keep it in the lines.
Bentley - Don't cut this at all. Tempting, but don't.
Follow through - the cones will slow you down, so don't hit them. On the left bend you can nip the cone in the grass but don't push it.
Bachrach - You can't cut the inside but you can hit the grass on the exit. Don't do this though, because all your passing chances will be on the inside.
Gambon - You can't cut the inside, despite the tire tracks tempting you. You can squeak on the outside of the paint lines if you must, but keep it clean across the line.
That's about all I've got for now. I did the challenge with nothing but ABS turned up, and the racing line on (though after a couple hours on the track, I really didn't need it anymore). Hope this helps!