I have a somewhat similar setup.
I have blades 5.3 and down which are my main sails I use on my 100 and 80 L freestyle and wave boards. However I still have a older 5.6 S-1 that I use and like. I only use this in light winds, what I like about it is rig it with a slightly tighter leech, its very stable clew first, and a very light sail for the wind range I use it in. i.e. I use it to improve my manoeuvring skills, otherwise I will get onto the 5.3 ASAP.
I like to reference the position of the batten above the boom without outhaul tension versus looseness of leech, particularly with the s-1 I have which are more sensitive to down haul, because of the repeatability of that approach.
If you just want to go fast in light winds, trade the 5.7 towards a 7.5M and ~70cm wide blaster, like your wise friend has