I just can't imagine how you can arrive at a figure of 15 points. It's not likely to be anything like that. But in an ideal world, where you had an unlimited budget, a new brilliant goalie would save some points, that's undeniable. But you'd have to spend an awful lot of money to buy a brilliant goalie.
In terms of points return on transfer money spent, a good striker would return far more points. Boro's weakness is scoring goals, and everytime the team fails to score, it's at least two points lost. The biggest scorers are almost always promoted to the Prem League.
With three goalies on the books it doesn't seem likely to me that Chris Wilder would want another.