Ulysseys,
With the 400 Axis arrows you are using right now, try going up in tip weight 145, 175, 200, 250 even 300 grains and see what happens? You will be reducing the dynamic spine and be observing the extremes range of what these arrows might do through this particular bow. I also have a 58” Super Shrew Deluxe 56#@27" and shoot Axis 400's through it at 28.5" BOP to trough of nock. There are too many things to play with to say whether it will improve your arrow flight or not, by changing the point weight, but it is a good starting point. Work with nock height, brace height, silencing location, your release technique and follow through, fletch size (number and configuration), etc. This is a never ending process in my IMO and a passion for me.
I would personally stick with the 400's, you should be able to get close to perfect flight with these and they are a stronger shaft than the 500's, hence more durable.
Good Luck
Kris