There are a couple of fluff pieces on the web that appear to indicate the George Forman Grills remove fat from foods, due to the tiled bed allowing draining:
In my opinion, both have the feel of echoing press releases from the manufacturer.
Neither compares the effectiveness of the tilted bed grill to a more traditional grilling method.
Consider griddling a hamburger on a flat griddle or in a flat style panini press, with or without raised grooves. Even without the grooves, as the hamburger cooks, it expresses fat and other liquid as the proteins in the meat tighten. With a fatty hamburger, this can be a considerable amount, even though it is not draining away.
The issue in expressed fat is not the tilt, but rather the squeezing effect the cooked proteins have, forcing the fat out of the food. This happens regardless of the angle of the bed.
Consider also that it is the contraction of the meat that is important in expressing the fat. If the fat were simply liquefied but the proteins not fully tightened, less or no fat is expressed. This can be seen in low temperature sous-vide cooking. Adhesion and capillary action would keep the fat in the meat otherwise.
The logical conclusion is that the tilt of the bed is irreverent in expressing the fat, but does help clear it away and collect it. This may have some minor effect on the amount of fat adhering to the outside of the food item, but is not the major effect.
You can argue whether the clearing of the fat away during cooking is a positive or negative for culinary quality, but it doesn't affect the amount of fat.