So that bow in your hands in the picture, is set at 52 lbs. Now for max speed build a lighter arrow. 52 times 5 equals 260 total lightest arrow weight. It hard to build an arrow that light but it could be done. that would give you maximum speed, however it won't give maximum Kinetic energy.

If you plan to hunt with 55lbs 5 times 55 is 275, which is the lightest you should use. I would go with an arrow that weighs in around 350 grains and I would use a small broadhead around one inch in diameter.

Still you stated speed of 220 for a 320 grain arrow doesn't seem right for that bow for some reason..

what was that bow's IBO rateing????