I can't really find any info on the Quatto shafts in particular. But the 2315 is the correct spine size for your specs, assuming a 27 inch arrow and 100 grain head.

The spine and weight are NOT the same thing. It is possible to have a weak spine, but the weight is fine. I think the bow is fine. You have to shoot an EXTREMELY light arrow to really hurt a bow.

ALOT of guys on here shoot 70lb rigs with arrows weighing in the 300's, which is light.

The number listed on arrow such as 340, 400, etc is the spine rating.

Now, the chart says that if you drop to 70lbs, you can use the 2215's, if you use a 27 inch arrow and 100 grains... I assume that is close to what you were using since you said the 2315's were recommended at 80lbs.

Don't worry about the bow.