I don't disagree with you that grey knights are an excellent shooting army, I disagree with you however it's the best codex. In fact the top ranking army at Adepticon 2012 was Necrons not grey knights and if more people were willing to paint up an entire guard army and all the vehicles you'd see guard at the top of the list and not grey knights.
http://bloodofkittens.com/blog/2012/...lts-and-lists/
Also grey knights would have to get to mid-field first, that's 2-3 turns off being shot at before you get to mid-field and then even when they did get to mid-field, hammerheads, devilfish and gunships if you're a smart player would have pulled out of that 24-30 inch range. Plus the suits get a jump back move and the range on the broadsides is 48 inches (actually may even be more). In fact I had a friend who took Tau to a tournament and he had all wins and one draw in Australia. Tau are lacking a lot compared to the new codexes (you can never compare an old codex to a recent codex - nids aside which was just an aberration) and if the rumours about the new codex come true Tau will be a very very good shotting army maybe even better than Guard.
Also funny how space wolves never seem to get mentioned as a good shooting army and you only mentioned IG, GK, DE and Necrons, a potential 25 str 8 missiles shooting at8 different targets a turn isn't anything to sneeze at.