Mine has the 'dual' drilled and tapped positions. I used the T3 and a GT3076 WG [popular choice for many]. And yes, in theory you could use the T4 footprint for a bigger turbo, but tI wouldn't like to port match the T4 footprint to the turbo [assuming the T4 has a larger throat]. To do that I would guess you'd firstly have to build up the outside with cast weld to strengthen and thicken the metal.
If you didn't do this I'm estimating/guessing you'd almost certainly break out, or thin the metal that much it would be too weak and fracture/burn away.
By the way, the limiting factor isn't the turbo size, there is plenty of room all round, its the turbo inlet size. I have a 100mm unit and had to cut back the front engine mount for clearance. I actually thought of refacing the mounting position ie cut the T3 mounting face back at an angle [say 5 degree's], this would swing the turbo mouth out a bit, but then you have the stud holes, so you would have to weld them up and retap square to the new face etc. I don't think this would have affected flow, the manifold casting [tubing] is still at a slight angle as it drops to the T3 face - but then I got lazy and left it. Now its on and I CBF, its working, I'm driving, thats it.