Ok, I just realized that Construct doesn't like it at all if you scale the text object! It drops the fps like crazy so I guess that it could probably cause other problems too.
I made a test capx showcasing the "zooming to the center of the window and not somewhere else" (if I may call it that) behavior. If you set the GUI layer's scale rate to 100 and try to scale the layout, the fps will fall like crazy!
I also tried the mouse.x("layer") expression that Sargas mentions, it works as it should! I don't have the energy to further look to the layer.canvasToLayerX(x) right now and it looks more complicated.
What would be very useful is to be able to specify the point of interest, in other words to have a variable defining the origin point upon which the transformations take place. Both with scaling and parallax scrolling. But this is an other topic https://www.scirra.com/forum/viewtopic.php?f=146&t=83989.