For the 2011 G0 155 your sails are quite small.
What is your weight in Kgs or lbs.
Your 465 mast (this is a very odd mast length as windsurfing masts are usually 370/400/420/460/490/520/550cm) is also several sizes too long for a 4.2m2 but
could be about right for the 5.7 m2 (420 is more likely the correct size).
The GO 155 could be a wonderful board to start out on if you are past the very
basic beginner stage where you would depend on a centerboard and rig steering.
Your Go 155 may have the fin slot in the center for a removable center fin. This
is a very good feature as you can use a center fin right at first, then put the plug in
and sail the GO 155 as a shortboard.
If you have experience with larger sails, the GO 155 can be sailed with any size that
suits your conditions up to about 10.0 m2.
What windspeed do you most often sail in?
Hope this helps,