The problem is that most pugs don´t fail because of 15% damage or healing etc.. they fail because the flaming shield 2nd boss in GB kills the healer and 2 dps in the first 5 seconds.... or the healer eats a ´blitz´on the first boss and dies...... or 3 people get insta-killed by the first boss in SC... or the tank gets insta killed on the third boss.... etc.
The number of times I´ve failed at a boss in a pug because of 15% is very low... Pug failures are always an on/off ínstant´situation... they are rarely a grind-down thing.