I don't know if you'll be able to get a consensus on this, or even if such a consensus would have any value. Some believe the grey ball have some meaning, others believe that the initial list is completely random and the grey balls simply are set-up so the first nine in the list get 5 grey balls, the last nine get 1 grey ball and there are always ten of each of the others. The BB's aren't likely to give out the actually information on this. At least they weren't willing to do so when the new system was introduced. And personally, I think it's probably best that way.
I generally do the group demonstration if there are enough 18 year olds in it to make it cost effective. This year I have 16 of the 19 players in the group demonstration who are 18 years old, so it is definitely worth the money. Last year I only had 7 and only two of those were of proper height for me to train, so I didn't bother.