Computers are improving quickly! On 9-9 board the game is already hard even for top pros. I thought that on 19-19 board it is still possible to beat computers on 6-7 stones handicap. But now I see that I am wrong. You can check recent games of bonobot (Crazy Stone). It recently reached 6d level on KGS (currently 5d) and beat his first 5d in even game.  As you can see from games list, bonobot has good results with 3d players, beat some 4d players and even one 5d.

Now I feel that the right handicap between top professionals and top computers is about 4-5 stones

Some information about bonobot (Crazy Stone):

Crazy Stone is a program by Remi Coulom which uses Monte Carlo Tree Search and pattern learning. Crazy Stone came second in the 2007 Computer Olympiad behind MoGo. 

In July 2007, a  match was played on KGS between Crazy Stone and KCC Igo, which Crazy Stone won 15-4. KCC Igo is considered the strongest commercial Go program.

On 2007-12-02, Crazy Stone won both of the KGS tournaments and the UEC Cup in Japan, ahead of MoGo in all three events. In 2008, Crazy Stone defended its UEC Cup title, defeating Many Faces of Go. Afterwards, it defeated Kaori Aoba, Japanese 4p, in a 7-stone handicap exhibition game. This gives Crazy Stone a claim to being the strongest program in the world.