Hi zobbo! Looking at your configuration, there are several values that differs from my setup.
As instance I use higher mass, lower friction slip, higher roll influence, higher engine force (but it is related to the mass, so it should be expected).
As for the other values, it looks like we use similar values (and here).
Actually, configuring Bullet Vehicles is a delicate process, since a lot of factors influence car’s behavior: wheels’ connection points, chassis’ shape, … I started from a working example, then I modified one parameter at a time. Moreover, looking at Bullet’s source code should explain some settings.