"On the other hand, it is quite likely this has already been tested numerous times over and I suspect the loft impact is negligible,(or the results would be made known) especially when you consider the average tripper may only spend a few weeks a year using it."


its likely that the "average tripper" won't notice a significant decrease in loft/warmth unitl several or many years of use.  Most of the comment I recall from backpacking forums were from thru-hikers on Appalacian Trail or whatever - some of thier comments were more or less that they noticed a definite loss of loft/warmtn after about a month of steady use on the trail.  

The "problem" with this issue is there are no "statistics" to look at - i.e., how much compression is too much; how many times is too much; how much loft was lost.  I've never precisely mesured the loft in any of my bags, so I have no way to tell if I have lost any loft - I doubt that 1 person in a million would have measured that precisely enough to quantify any loss of loft.  So all the people who say they haven't had any loss can't really prove that either.  So its all subjective - I go by the logic, that it can't be good for the bag, and may be bad, do I don't want to do it if I don't have to.

Out of curiosity, I looked at the Western Mountaineering, Feathered Friends, Marmot and Montbell websites (all top quality down bag makers) - they were all mute regarding compresion sacks - nothing said for or against (though agian, none of these companies give you a compression sack with thier bags, but rather a simple stuff sack.