The viewfinder will show what the camera captures, or at least almost what it captures.
Using a lens that supports a larger image circle will not let the camera capture a larger image area, or change the relation between the sensor and the viewfinder.
Some cameras have a viewfinder that shows 100% of the captured images, but it's common that it shows slightly less because it takes very precise calibration of the viewfinder optics to reach 100%.
The live view will naturally show 100% of the captured image, as there is no optical calibration required to acheive that. (Also cameras with a digital viewfinder will show 100% of the captured image.)
As a comparison, an film camera typically shows a bit less than 100% of the captured image by purpose, to represent the final printed result. Commonly the print process masks or crops the image slightly, so that it doesn't show the uneven edge of the captured image.