You might be hard-pressed to find any true research or scientific data on this subject. It likely does not exist.
It would be nice to think that the NFHS had such data available to them when they decided to make this rule change. They did not.
According to Mary Struckhoff, NFHS Assistant Director and co-editor of the NFHS Rule Book, the NFHS had absolutely zero scientific information available to them when they approved the use of metal cleats. She revealed this in a speech given at the ASA National Convention last November.
When the 2008 high school rule changes were first announced last summer, a press release gave this justification for the new rule:
"Metal cleats provide better traction for athletes on nearly all surfaces, including hard, wet or under-maintained fields," said Cindy Simmons, chair of the NFHS Softball Rules Committee and assistant executive director of the Oregon School Activities Association. "They also help prevent slippage on the bases, especially home plate."
Last month, the NFHS posted an article from "Fastpitch Delivery", a publication of the National Fastpitch Coaches Association, on their web site that deals with the transition to metal cleats that college coaches have dealt with for years. While not a scientific study, there are many anecdotal accounts of what these coaches have experienced, and some of that might translate to what we can expect to see in high school softball this year.
(Link to article:
http://www.nfhs.org/core/contentmanager/uploads/Transitioning_to_metal_cleats.pdf )
I find it interesting that only one of the coaches quoted makes any mention of injury to the knee as a result of the cleats. That coach does not allow any of his players who might be recovering from a previous knee injury to use metal, as he seems to feel that the extra traction increases the possibility of reinjury to the weakened joint.
I would also note that, in general, college players will be bigger, stronger, more skilled and better trained than the typical high school player.
It appears that the 2008 season will be the test-bed for how this new rule plays out- and the high school players will be the guinea pigs.