Pattycake, if your recipe is tried and true with the 5 eggs and 2 yolks, I'd stick with what the recipe calls for. The egg yolks are going to give you a denser cake than if you had extra whites in there. My standard cheesecake recipe calls for more whites than yolks, but I bet the balance of other ingredients is different than yours, too.
There are a ton of things you can do with egg whites: souffles, egg white omlettes, and if you don't want to use them right away, you can freeze them and use them later (a great way to do this is to put them in icecube trays.).
Hope this helps! Good luck with your baking!
-A balanced diet is a cookie in each hand