Your advantage at high counts is a very complex thing.
Your overall advantage is the sum of:
- The probability a certain hand will show up.
- The probabilities of outcomes of your choices (hit, stand, double, split).
- The value of the outcomes.
Changing the count will actually affect all three, and sometimes in opposite directions. For instance, at high counts, hard 11's are actually less likely, since they're formed with two small cards. But the chances of doubling to 21 goes up, and the value of a 21 also goes up. Likewise, the probability of getting a soft hand goes up, but the value of soft doubles actually decrease.
Most of your value comes from the fact that blackjacks are more likely at high counts and you get paid 3:2 for them; hard hands (excluding 10-10)increase slightly in value (mostly from player 12-16 vs. dealer 2-6); soft hands actually decrease in value; and paired hands increase greatly in value (mostly from 10-10 being more likely).