A 306 is basically a 302 block that has been bored .030 over. Just shaving off some of the worn area. Doesn't really help for power. If you want power have to go for the 331 or 347.
From the rings, it's just normal wear. Usually on your 1st rebuild you'll go .030 over to true up the cylinders. Isn't worth it at all as a performance mod. I'm sure it will add some power after it's all said and done cause you get to put new pistons in and you are adding a few cubes.