I'm sure all the above technical info is absolutely correct. However for us less technically minded it was explained to me in simpler terms. If you look at these problems from the extreme then it often becomes simpler. Imagine you have a 200kt TAS and you were flying into a 200kt headwind. Your SGR would be 0 therefore, by increasing your TAS to 205 kts you will be increasing your SGR. Obviously at higher TAS it is not as significant. On older Boeings i think the rule of thumb was not to worry about it unless the H/W was above 200kt. Now days i just stick the cost index in the computer and pull out the crossword.