It seems as though Ford intentionally uses a relatively small drill (i.e., smaller than 15mm) to accommodate manufacturing tolerances for ensuring that the drilled portion of the passage falls within the perimeter of the cast-in portion of the passage. As the position of the revised holes can be located to a much higher degree of precision in a custom forming operation, the exterior wall of the revised hole can be formed to be completely concentric with the 15mm OD cast-in portion of the oil return passage.
The thinnest wall section surrounding the passages is nominally .300", which is measured at the 15mm cast-in portion of the return passage adjacent to the head gasket surface. So, properly opening up the drilled portion of the passage to 15mm will only result in the same wall thickness at the drilled portion as already exists at the cast-in portion of the oil return passage. There are no cavities adjacent to the drilled portion of the oil return passages, only adjacent to the cast-in portion of the oil return passages.
It should be noted that certain approaches to porting the exhaust ports may cause some wall thickness issues between the exhaust port and the oil return passage. Those that intend on open up the exhaust port dramatically will want to confirm that there is adequate wall thickness between the exhaust ort and the oil return passage to enable enlarging the oil return passage. This not so much a concern when the OE angle of the exhaust port wall is maintained. But, when the taper of the side walls of the port are removed (e.g., via a vertical plunge with an end mill), wall thickness with engarged oil return passages should be verified.
Below are a few pictures of the enlarged passages and some illustrations showing the before/after oil passage configurations.
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