Unfortunately, no it doesn't, because there are typically other factors that influence the image circle - for example, the barrel of the lens can cause vignetting.
The Canon 200-400mm lens has an inbuilt 1.4x converter, and I've tested this lens on a full frame MF back with the Hartblei HCam.
Whilst the image circle does increase somewhat, it's certainly not by a factor of 1.4x.
Here's the lens at 400mm without the converter -
And here it is with the converter dropped in -