I have to agree with Steve here, as the 40 cm Lessacher Duo Weed is pretty good on wide formula type boards.
Also, for James, the new GO 155 is only 85 cm wide (almost like the original GO which was 83 cm wide).
Here's the specs (same as the Futura 155) except for the weight due to the heavier Tufskin construction.
GO 155 155 250 85.0 58.9
So, I think the 36 cm Lessacher Duo, a 39 cm Tangent Dynamics Reaper, or a True Ames Shallow Water Weed 39-44 cm will do a very good job on the Go/Futura 155.
I guess the choices are partly dependent on the severity of the weed conditions
and rig size. For rigs larger than 9.0, then a really large (more than 42 cm) weed fin
becomes a good idea as you want the extra span and area to get the best light wind
The Lessacher Duo sheds pretty much all weeds due to the asymmetical design.
The Tangent Reapers handle all but the weediest conditions and the True Ames
Shallow Water do just as well as well.
The True Ames and Tangent Reapers are progressive rake designs that feature a ramp, then a progressive rake to a slightly more vertical LE angle.
With more chord width at the tip of the fin, they give the same (or better) performance
on wider boards, using a shorter fin span (depth), and they do not stick way out behind the board like more tradition 45-50 deg. LE weed fins.
Hope this helps,