I guess I can wrap this thread up by saying I've done a deeper dive on some of the internet posts that claim performance problems with the HierarchyID datatype and its related methods. So far, all of them have turned out to be "false alarms" where certain (bad) programming practices, such as the use of non-SARGable search predicates, were used that would slow down any query and not just those related to the HierarchyID.

I haven't given up the search for actual performance problems caused by the use of the HierarchyID datatype or related methods, but I'm going to turn my efforts more to comparing performance between the HierarchyID methods and Nested Set methods.

Thanks to the good folks who posted on this thread and, believe it or not, thanks to the folks that didn't... it kind of shows that not a whole lot of people have to work with Hierarchies and the ones that are, simply aren't having performance problems with their hierarchies, are not working with large hierarchies, or are simply not aware that they may have a performance problem.