light winds and older sails ...
A full (baggy) sail is OK for ''acceleration'' and ''power-up'' settings .... needed for sailing in ''left-over'' chop, power boat chop, etc. in very light winds. However once the boat begins to reach its max. speed (for the at-the-time-conditions), a flatter shape in light winds usually is best to prevent flow separation - especially on the lee side and especially near the leech. Once a boat completes its acceleration, the apparent wind and the stream flow across the sail becomes faster and then usually the fuller shape is bad. Once the acceleration phase is complete usuallly a much flatter shape is needed to prevent the deadly ''flow separation''. My racing was/is primarily on ILYA scows and those boats do best to ''maintain speed with board-flat sails in extra light winds once you''ve accelerated out of a tack / gybe. If you apply tell tales, expecially in ''Gentry-Tuft'' fashion - at the luff, leech and foot - you can SEE the effect.
So typically you need a fuller draft to accelerate and get ''moving'' but then need to ''shift'' to a flatter shape to gain and maintain max. speed. A short course that involves LOTS of tacks and gybes maybe would benefit from keeping a ''bagged'' shape .... but not usually as you have to ''keep shifting gears'' to consistantly win. Just keeping a baggy shape will let you accelerate quickly but usually wont be able to reach the max. speed potential for the conditions - due to flow separation. Your tell-tales will show you.