% of spend increase doesn't coorelate that well to % of increased visitors. It depends a lot on positioning. In many industries, as positions go up, the bids go up even higher, so a 10% higher spend could mean only a 5% increase in traffic.
You'll often see large bid gaps between 1-3, 4-6, etc (depending on how they are syndicated on yahoo).
You're better off looking at the Y page, where your ads are displayed, and then finding a position that seems to offer you a good price, position, and visibility.