I don't think any of the top FAs will sign with Boston until Ainge goes out and acquires another star to put alongside Rondo first.
Chicago and Houston are ready to challenge for NBA Championship right away. Boston are not and are thus not competitive from a basketball point of view with the best offers on the market. Ainge needs to get another star in here to make them competitive.
So I would say there is no point in creating cap space unless you are targeting one of the second tier stars like Greg Monroe, Eric Bledsoe, Lance Stephenson, Luol Deng, Gordon Hayward, Chandler Parsons or Pau Gasol ... because the A listers are not coming.