(M)XL's are still positive engagement belts, they're trapezoidal and at low speeds tend to have wedge engagement at the root of the tooth. The big benefit you get with curvilinear belts (HTD GT2 etc) is under higher loads and higher tension. On small drives using a low installation tension ,small pulleys, light loads, and low teeth in mesh (like the stock shapeoko) Curvi and trappy belts have about the same performance. At higher tension and on bigger pulleys Cruvi belts are significantly better than trappy belts.
So IMHO if you're extend the load handing capabilities of your mini mill already, go ahead and add the GT2 belts. It's just a sprocket change.