This issue has been raised a few times here when the discussion is around long drive shafts though I don't recall seeing it in an OD versus rear ratio discussion.
In general the modifications talked about here reduce drive shaft RPM at any give road speed and therefore most of us are not in any real danger of turning the drive shaft too fast.
When someone suggests replacing a two piece shaft with a long single drive shaft I think it becomes a very important concern.
Standard practice for OEM drive shafts has long been to design them with a larger margin of safety. I don't recall the number, but I can see the engineering text with those numbers from where I am sitting...
A related discussion that I have not seen here is the parasitic power consumption of an OD versus a direct drive and the impact that has on performance and economy. My own feeling is that most of us would never be able to measure the difference.
