Question about Astrologian and buffing in 8-man raid.

[mosaicex]

Except in very exceptional cases, you'll want to give it to the whole group. Consider the following distribution, top-DPS of 2000, 2 DPS at 1000, 1 DPS at 800, 2 tanks at 700, 1 healer at 300.
Imgur hates me so you just get numbers.
Even for this lopsided distribution, the total dps is 6500, 10% would then be 650, but adding 30% to the top dps would only be 600. And that gap only widens as you have a more even dps distribution.

You seemed to ignore the fact that cards buff do not last the whole duty duration. So no, an AoE Balance wouldn't suddenly buff the party damage by 10%. Your example is really flawed.