Oldish monohulls with stiff masts often go with a deep mainsail in light winds becouse they run out of better options. Much of the sailing litterature is written with these boats in mind.
On a cat my experience is that flat is fast in light winds. Crank downhaul and sheet in. This was confirmed by a former olympic Tornado sailor as well if that counts for anything.
In light winds there is not enough energy in the wind to make it attach to the curve of the sail if we run with the medium wind settings. By flattening the wind dont separate off the sail causing energy loss + drag. Just look at the telltales, over flatten and then release a bit. Separation is pretty easy to spot with telltales at the leech and max draft.
Twist is overrated in cats
I believe most sail with not enough downhaul and not enough sheet on in most conditions..
Hooking the leech is a no-no in all conditions. Leech telltales again helps to avoid this but the specific trim settings depends on the mast pre-bend, how much downhaul is used and lastly how hard the mainsheet is set.
Light wind and chop, then it gets interesting.
I miss our Tornado..