Here's some numbers for what I find to be a very relevant comparison, Nic Batum 2 years ago.
1) a useful link for a rough approximation of the dollar cost of a win. With free agents, it works out to somewhere between $1.5-$1.86M per win
http://wagesofwins.com/2012/07/09/explaining-the-nba-cap-and-the-value-of-a-win/
2) Nic Batum was signed to a 4 year/$46M offer sheet based on potential coming off a season in which he had 4.8 win shares. Using the math, this means they would have projected him to generate about 6.76 win shares per year over the life of the contract (using $1.7M per win)
3) He had win shares of 5.8 and 7.9 over the first two years of the contract.
4) Gordon Hayward's best year, when he wasn't the focal point, was 5.4 wins.
5) If Gordon gets 15 million a year, the Cavs would be projecting his win shares to average 8.8 over the life of the deal.
6) list of players who didn't average 8.8 win shares over the last 4 years: Aldridge, Horford, Millsap, Westbrook, Ginobili, David Lee, Ibaka and there's more.
No way do I want to give Gordon the max.