Construct appears to use textures for the black borders, the amount depending on the amount covered. That said, I know people often here talk about keeping vram usage to a minimum, but 14 mb of vram really isn't that much. Any even remotely recent computer should be able to manage that and plenty more no problem.
Also, do you have any full screen canvases or layer effects? That could affect vram usage.
Otherwise, I guess my best recommendation would be to make the layout size always bigger than the screen resolution and make the pillarboxing yourself.
Edit - there are workarounds for is object on screen (check distance) and the UI (have a camera object, set position of ui elements to it + the offset - there's probably a better way than that but I'm not sure of any others atm. You could even make a ui family with xoffset and yoffset variables and set the offset at the beginning of the layout if you don't want to have to hand code the offsets, etc).