using an index worst performances

Hi all,
I'm tring to optimize the following query:

http://rafb.net/paste/results/YdO9vM69.html

as you can see from the explain after defining the
index the performance is worst.

If I raise the default_statistic_target to 200
then the performance are worst then before:

Without index: 1.140 ms
With index: 1.400 ms
With default_statistic_targer = 200: 1.800 ms

tought anyone ?

Regards
Gaetano Mendola