Definitely a lightweight sailor on overpowered 7.7V8 (pretty grunty sail as well) will be overfinned on iS105 with the 40cm option. Even at a svelte +/- 200lbs I'm already on 34cm in iS101 powered (or windier+) 7.6m conditions.
Basically, use the big (40cm) option for marginal to light 7.7m, or lighter wind/bigger sail(8+m) conditions, where it will work well, but once the action starts get onto the 34 asap for higher top speed ..and better control. (Note : if you're struggling with spinout on 34/7.7m V8) then it's likely the sail tune in those conditions (add more down/out haul and/or variable outhaul) and/or the chop is hindering your boardspeed, so go for a bit more speed bearing off or across the wind before you start to pinch up hard.
So to the fast stuff : yes , definitely a good fast 32cm is an asset in fast 6 or 5,5m conditions ; 32 will work well even in well powered 6.6 conditions and you can get away with 30 (lighter rider) in 5.5.
Going smaller than 30 is a bit marginal except in pure speed (flat water and high board speed) conditions as the iS105 needs a certain amount of fin to help maintain a flat trim across the tail.
Yes, a one fin "31" option will work well too, but with the usual provisos that not all fins size "performance" wise in proportion to their mechanical "cm" size,
(brand/model X 34 = performance size of brand/model Y 32 etc)
so when you're accurately selecting alternate fin/s, try and be sure to compare "performance" size rather than just mechanical size.
Selection of make/brand (etc) is usually influenced significantly by what is available at a practical level in your location (or accessible in practical way by phone/web/mail etc).
For a hi speed small fin to push the limits, it's good advice to make sure whatever you end up with, make sure it's a good one
Cheers ~ Ian