The change in horsepower due to the change in compression ratio is relative but not directly proportional. That is to say that a change from 8:1 to 9:1 will give you a larger increase than would the change from 13:1 to 14:1.
I seem to recall for every one point change around 7:1, that is the change from 7:1 to 8:1, would be slightly more than a 3% power increase. Once you get up to around 13:1, that same one point change is only good for about a 1.5% power increase.
The rule of thumb for the compression ratios run in most street engines is: for every point change in the compression ratio your power output will change by 2%. Using this rule of thumb on an engine that produces 100hp, every 1 point change in compression ratio will result in approx. a 2hp change in output.