Remember that you are creating a longer "Lever" the further you put the cutting head out away from the Z plate. Too much and you'll bind your axis under cutting forces.
You're X travel is not really dictated by where your Z is located. (unless of course it hangs off the table at one end. ) It's dictated by the the size of the Gantry base. No matter where your spindle in located within the gantry's footprint, travel is the same as long as the Z is "within" the footprint. What you presented doesn't really gain you anything. But it does force you into having to make a really high gantry with a limited Z clearance. Which could be a poor trade off in gantry stability.