Stick with the 2315's. There is no other arrow with a .015 wall thickness that is lighter than 2315 and also properly spined for your bow. If you want lighter and still have durability, you have to go carbon.

A rule of thumb about aluminum arrows might help you. The size numbers specifically spell out the shaft's outside diameter and wall thickness but also can be used to kinda get an idea of what two different arrows would be like. The first two numbers are more related to spine, like a 2315 is spined heavier than a 2115. The second two numbers are more related to weight. Like a 2219 is heavier than a 2215. It's not 100% foolproof, but it generally holds true.

So, if you're looking for a lighter aluminum arrow, you have to look at arrow sizes with the second two numbers lower than the one you've got. On the other hand, one rule of thumb that always holds true for aluminums is, the smaller the second two numbers, the less durable the arrow is.