One thing to keep in mind is the spine charts and software to pick a spine is a suggested STARTING point. Two supposedly identicle bows in two different shooters hands could "like" different spined arrows.

As mentioned there are ways to take a big arrow and weaken its spine, any weight you put on the front end will in effect weaken the spine, and any weight you take off the back end will weaken the spine, so:
-heavier insert or BH
- lighter fletch (feathers over vanes for example)

At 20 yds and 300+ fps, you will need it to stablize quickly for max penetration (but keep in mine we could be talking about how hard you have to pull to get that pass through out of the dirt )

If you want the bigger/heavier arrow, go with big, thick walled alumns or some of the carbons that are higher gpi. Then leave it 3-4 inches longer than normal and toss in a big head or heavy insert.

If you want to see how stable/unstable it is, do as suggested above, shoot without the fletch on it (for a perfect match, wrap an equal weight of tape around the tail end to get FOC the same). This will show how much work the fletch is doing.

When your form is dead on, the spine being stiff doesn't matter as much, but it does matter a little.