Someone may correct me here....but below are my suggestions;
-Try and do a full boil if possible
-If you're going to add water, add cold water (adding water or not, the idea is to get the wort down to pitching temps quickly)
-I wouldn't let the wort get all the way down in temp, because the cold water will speed things up once you add it
-Logistically, I would cool the wort down to say 100 degrees, siphon this into your fermentor and then pour in your cold water up to the 5 gallon mark. If you're adding another 2-2.5 gallons of cold water, that water will be down to pitching temps almost immediately.