RE: A smaller board for bigger conditions?
Yes, a smaller board will handle the chop a little better than your GO 170. but if you really get your GO "trimmed and tuned" you should be able to sail it farily comfortably up to around 20 knots.
When you get to 20 knots, then you would probably want a board around 100 liters or smaller.
Older Carve 99 or 101, Kombat 96 or 107, maybe an Isonic.
101 or 105.
As far as the formula racers go, first, they are racing, or training to race so they are willing to put up with a higher degree of "difficulty" to be fast, and hopefully win races.
So, they sail the huge sails and wide boards as this is what it takes to go very high upwind and downwind.
You may be onto the "secret" here with formula boards. The shorter length, larger fin and footstrap offset, combine to make it easier to control the "pitch axis" trim of the board.
This makes going at very high and low angles, in chop, alot easier to do.
Hope this helps,