I would think a longer arrow would actually reduce penetration because of the added drag going through the animal.

I don't think arrow length has anything to do with accuracy as long as they are spined correctly and tuned well. If it made a difference every target shooter would be shooting really long arrows to improve their scores. And the only time I see them shoot a longer arrow is when they want to shoot a larger diameter arrow to increase line cutting ability. Depending on the bow you shoot you may need to add quite a bit of tip weight and/or shoot longer arrows in order to break the spine down to match your set up.

The only thing a longer than normal arrow would do is let you shoot a heavier arrow. What it would do is make picking the correct arrow a pain in the but.

And another thing to consider is that arrow specs are more accurate for shorter arrows, especially with carbon arrows.

Paul