My understanding of the question mark with regard to balance and radiation is this: radiation treatment doesn't usually change balance one way or the other. Surgery does change it, because they usually cut the balance nerve on the affected side.
For some patients, the balance nerve goes out over time anyway, and the brain readjusts to using the nerve on the other side. I think 76% weakness suggests that you are headed in that direction already. I had caloric testing, and it showed only a very slight weakness on my AN side; the balance nerve has not been an issue for me.
There are cases where the balance nerve is out of whack, but it doesn't fade away enough on its own, which can lead to severe vertigo issues. Surgery will fix that condition by cutting the nerve; radiation won't change it. That is the situation where surgery can be a better choice of treatment.
If you are not having severe vertigo, and your balance nerve on the AN side is already 76% gone, then your brain is already making the adjustment to one-sided balance. At 1 cm, you can probably wait a little while, and see where the balance situation is headed. If you wind up readjusted to one-sided balance, then radiation is still a good choice. If you wind up stuck with nasty bouts of vertigo (the 3 hours flat on your back kind), then radiation might not be the best choice.
That's my understanding of the situation, anyway. A good place to ask doctors a question like that would be on the cyberknife forum, at
http://www.cyberknifesupport.org/forum/.
Good luck with your quest,
Steve