12.5 Stone=175 lbs.= 79.4 Kg. so, you are kinda right in the ideal weight range that Starboard
uses for board and fin design. This is good!
I think the GO 151 might be a bit better as it will have the option to use a center fin while you are learning
to transition from the 225 liter with a centerboard/daggerboard to the 151 liter with no center fin.
This could really help your transition.
What you will find you need to do (without the center fin) (until you get fully planing with larger sails (what sail size are you thinking of buying?)) is to stand enough off center on the upwind side of your new board to get the bottom inclined a little bit upwind rail lower than the downwind rail so the board will carve upwind on the rockerline in the bottom of the board.
Once you are fully planing, then getting upwind should not be a problem.
Right at first, you are going to notice a HUGE difference between the 225 liter longboard and the GO 155.
The longboard with the centerboard down/deployed allowed you to use "rig steering" to go upwind.
If you rake the rig back a bit, the board turns upwind as the power from the sail gets behind the lift from the centerboard.
With no center fin or centerboard, the GO 151 is going to be completely "foot steering".
Having the small center fin (you can change the size to suit if you like) will give you some rig steering capability until you get the feel for foot steering.
So..... perhaps an easier transition, but as soon as you are ready, put the plug in the center fin slot and
sail the GO 151 like a true shortboard, using weight off center when slogging (almost a form of foot steering really) and then completely foot steering when you get planing.
Hope this helps,