Sorry if posted before. I recall that he posted concepts and images of the models but now you can load the models up in the browser and rotate and look at them. Very sad we didn't get these in the game.

Twister-
https://www.artstation.com/artwork/QDwG3
 
Crazy 8-
https://www.artstation.com/artwork/rDJ3G

Yellow Jacket-
https://www.artstation.com/artwork/0bO8G

Maps-
https://www.artstation.com/artwork/l4bq5

I don't like how orange Crazy 8 is. Should be red.
Twister looks about the same as it always has in the series.
According to Barker had the DLC existed it would have been them 2 vehicles & Yellow Jacket...
But what about Mr. Slam? Was Mr. Slam not considered for the cancelled DLC? I can't understand why they would not have put him in, considering Mr. Slam's popularity.

As for Spectre I believe that vehicle simply became Kamikaze,
and Pit Viper, who knows if that model actually exists. Jaffe claims there was a model made for Pit Viper but there's no evidence of that on the internet.
There is a hidden easter egg in the game that suggests the vehicle was considered... in Sunsprings, CA if you look on the side of the school buses on the map you will find the name "Fortin" in reference to the fake name Pit Viper's driver used (Angela Fortin).

Really odd how poorly thought out the vehicle roster was. For instance, Brimstone wasn't even considered for the roster yet Preacher is in the game.
Roadkill & Death Warrant were too similar. Kamikaze & Crimson Fury were too similar. They could have cut one or the other in those pairs & included a Crazy 8 or Yellow Jacket or Twister.
Hammerhead was another mistake, wasted as a non-playable vehicle when HH (or Slayer) should be in the playable roster.

Did Shylow Barker do Mr. Slam or was it Mark Wahlquist?

https://vignette.wikia.nocookie.net/twistedmetal/images/f/f5/Orig_mr_slamm01_923...

Mr. Slam is absent from Barker's ArtStation page.
The art seen on the Fandom TM Wiki lacks the info. Did Jaffe mention that detail? The only thing I dug up was an old TMA post that had a dead link that seemed to suggest it originated from Mark Wahlquist and that he made the model for Mr. Slam (or Slamm if he went with that spelling).
However, Mark was the Environment Artist of Twisted Metal PS3, so I find it odd he would have done Mr. Slam's model, although were that the case, it would explain Slam's absence from Barker's art page. 
Anyway, on digging further I found Mark Wahlquist's new portfolio site which features Twisted Metal PS3 environment model art, but no Mr. Slam.

Funny enough he thinks TMX was the seventh Twisted Metal game, when (excluding TMBO & TMHOETE) it would be the 8th, unless one dismisses TMSB as a spinoff.

Edit: His resume says
• Worked on the game 'Twisted Metal' doing Environments, Vehicles, Weapons, and Props. 

So I guess maybe he did do Mr. Slam.


http://www.markwahlquist.com/portfolio.html

"TWISTED METAL
This was the seventh Twisted Metal game, it came out in 2012 and was a PlayStation 3 exclusive.  These are a few of the buildings and areas I worked on."