RE: GO or Carve? From 129 to 170?
To some degree, I would agree with what "people have told me" but it's not quite that simple.
Look at the width of the different GO boards. That's the key.
Wider GO's will plane earlier, but when the wind gets to Bft F5 the width begins to become a liability.
Narrower GO boards will be a little more comfortable in Bft F 5 but will
not plane in Bft F3 (even high Bft F3) for a 177 lb. sailor.
Since I weigh almost the same as you do, here's where I would put the numbers on this.
The GO 155 will give up about 2 knots of early planing with an 8.5 m2 rig over the GO 170, but you will get about 2-3 knots of higher wind performance before the GO 155 starts to have the same "higher wind"
issues as the GO 170.
Your idea to get the GO 155 (I think this is a little small for a sailor your size) and keep your poly longboard for the light wind days is a good plan.
Or get a Carve 133/122 and get even a bit more medium wind performance and a true free ride feel.
Overall, the width of the GO 170/155 is going to make them a little eaiser to sail in 2-3 knots higher windspeed when compared to a large or mid sized Carve.
This is not something that very many people have figured out.
At around 20 knots (Bft F5) mid size and large Carves will suffer from the same "it's beginning to feel too big" issues as the GO 155/170, but the wider footstrap offset and larger fin on the GO boards will give you a little more range with good control.
This is mostly due to the surface conditions of the water than with the wind speed.
So, if where you plan to sail has big disorganized chop at 20 knots, you won't be comfortable. But on the other hand, if you have fairly flat water at 20 knots, then the GO and Carve will be fun and comfortable with smaller 5.5-6.5 m2 rigs in 17-20 knots.
If you have the big chop with lighter winds, then I think you will be looking for a 95-115 liter board real soon.
Hope this helps,