I don't think anyone has specifically looked at that from the sense you are coming from - they generally have compared breakdown rates at the same facility before and after synthetic installation without separating out where horses are training or shipping in from (Europe, Australia). As you know training locations vary more in countries other than the USA.
You might consider that the non-USA studies are nearly all turf (no dirt), so it's virtually all turf to synthetic.
Go here:
http://www.ncbi.nlm.nih.gov/sites/entrez
Enter as your search terms thoroughbred equine
That will get you a couple thousand hits (limited to recent), then you can narrow (don't narrow too specifically initially). You also can do an advanced search as alot of that data is now older than 5 years so is falling off the "current" screen and doesn't come up unless you search diligently (I was trying to find the NYRA data on breakdowns at Aque-Sar-Bel from the 1980's)