One perk to a sail that weight, for your intended purpose, is that you can have a luff wire sewn into the sail. Or a Kevlar or Spectra webbing luff tape stitched on.
That way, you can set it flying, especially from something like a Screecher or Staysail furling
unit. So that it's easy to pull out & hoist, without having to remove or attach anything to a stay. Which is particularly handy if you have roller furling
The free flying luff, definitely has some perks. As it gives you more latitude to tune the shape of the sail; based on wind
conditions, & how high or deep you're trying to sail.
Also, with that weight of cloth, & a built in luff wire, the sail is kind of like a poor man's Code 0. Especially as, when you tighten up the luff, you can fly it upwind, to about 050 AWA+/-, until there's truly enough wind to properly switch over to your primary upwind headsail.
That typically being a 135% or so, jib
, set on the furler
(or hanks). Which comes into it's own when the wind goes North of 7kts or so.
Honestly, it's quite a common setup. The Pardey's used something akin to such, & did Beth Leonard & Evans Starzinger. http://www.bethandevans.com/sail_combinations.htm
I even had the sailmaker
rig one up for me from a light #1 (155% jib), as a cheap
fix for light air on a 40' racer
that I had for a while. It's purpose was to augment the 125% jib
which lived on the furler
Though as another poster already said, there are definitely times when it's best to get the mainsail
board flat, along with hoisting a superlight 100% headsail (Windseeker), also tuned board flat.
Albeit, in a pinch, you can use your #3 (100%) or similar, in lieu of a Windseeker.
Such a setup (combined with a flat, hard sheeted in Main) uses the boat's rocking to actually extract some energy from the seas & what little wind there is, in order to drive the boat. And in some light air conditions is a far faster rig, than if you were to hoist a 200% jib.