At 80 kg and with biggest sail of 8m how much if any benefits are there in going for higher volume board? I have a super cruiser foil and a gtr ( 1000 front wing ) . Thanks

Do you need to slog out to a distant windline?
Does the breeze at your spot tend to shut off or go really low on lightwind days?
Do you need the ability to head upwind even while slogging?
Are you trying to foil even in the lightest 7-12 days?
Do you plan to foil when the wind hits 15-25?

grab the bigger board, always

If you are wanting to carry an 8m sail, you will want the additional tail width of the 150, regardless of your weight.

The 150 is very similar to the iqfoil youth (and the slalom one, per brochure). It can carry up to 9 m with no problem. I am guessing the 125 might be a tad small for that task. I haven't tried them though.

I have the SB 125 freeride and the Supercruiser. In combination with my largest sail, a 6.4, I would not have wanted a bigger board. In your case, Im not sure. Im 78kg and I need to slog quite a bit.

the 150 be the go im 85kg used up to a 7.5 meter sail gt foil ,floats along so sweet and tacks with out the nose sinking the main reason I bought the 150 the 125 might sink if you want to tack. but I don't know how the 150 feels flying I havnt worked that bit out yet

Steview, As you are running the Starboard Super cruiser and Gtr with the 1000 front wing, I would lean towards the 150L board. I am a fraction lighter than you and have the same setup. The only challenge is that the supercruiser foils prefers a narrower board as your feet are much more inboard and your weight much more over the board as opposed to leaning out. If you only had the supercruiser i would say the 125L freeride, but since you have the GTR with the 1000 front wing, you benefit with going with 150L which can still work with the supercruiser even if not perfect. I also highly recommend you adding the 115 fuselage to your foil quiver as this will significantly increase the lightwind foiling performance when combined with the 1000 front wing. With the supercruiser setup, I also enjoyed adding the 2000 oceansurf front wing. In my opinion this made an even more super cruisy setup. :)

I'm only 75 kg, and when I slog my 122 Naish out to the windline...and back later, the deck from just behind sail mast track is fully awash with water, while the rear straps are half submerged.
That's not a lot of excess flotation, with a 5.2 sail.

If light wind is the goal then extra volume really helps, especially with faster race foils.

More important than early planing is slogging comfort. If you have to slog often then a bigger board will make that an acceptable interlude whilst the smaller board will be a real frustration

For light wind, bigger is always better. You plane quicker and take off earlier. Small boards are good for freestyle. You DO NOT need to apply windsurfing board sizes here. Big works, and works well.