RE: Go 155
Yes, an increase in sail size, normally moves the CE of the sail back away from the mast foot, and at the same time closer to the fin.
When you rake the rig back one of the things that happens is that the CE (Center of Effort) on the sail moves back and is supposed to balance over the CLR (Center of Lateral Resistance) of the fin.
This is one reason why sailors normally move the mast foot forward a bit as their sail size increases.
The idea here is to strike a balance between the resistance of the fin and the power in the sail.
When you get everything prefectly balanced, sailing is soooo easy. Everything seems to work. No spinout, no "upwind or downwind tendancies", higher speeds with improved control become easy to achieve (up to a point of course).
So, it's very likely the larger 7.5 rig, the definitely too wide for a 32 cm fin GO 155, and your skills are all contributing factors.
Harness line balance/positioning, footstrap positioning, boom height, type of harness, stance, all are contributing factors in the overall "balance" of your board and rig.
It's always necessary to try changing things (one at a time) to see if you can improve the overall balance.
So, basically, your "spin out" problems are most likely the result of something not balancing. Could be the fin/board combination, could be the fin/rig combination, or simply a technique issue like getting your skill level up to where you are simply pushing too hard for the size of the fin.
Hope this helps,