What about the overhead of the comments themselves?  This if often overlooked.  It's my understanding that SQL does load comments (even full-line/multi-line comments) into the proc cache.  So lots of comments = lots of extra proc cache space that could be used for something else. 

Thus, I prefer carefully chosen variable names with minimal comments on areas that are difficult or obscure.  Hopefully at some point SQL will get better internal code and simply drop comments from the code as it is loading the proc cache.