... Actually I think it was Jeff H who said there was no difference, and I'd be concerned about disagreeing there too..
Jeff H was talking about mainsails with an attached shelf-foot
. They were previously used mostly by racers, and many cruisers didn't know there was such a thing. The shelf foot enabled a racer to create a very deep draft for very light air. A mainsail with a shelf foot made the boat faster in light air, but it didn't make it any faster in 8-15 kt winds. The attached foot mainsails that came with most sailboats in the past didn't have such a deep shelf foot. A modern, loose-footed mainsail enables the sail to be given a very deep draft, by easing the outhaul, similar to the shelf-footed mainsails. Thus, it gives you the advantages, in light air, that you could only have gotten in the past with a shelf-footed mainsail. But, when trimmed for 8-15 kt winds, a loose-footed mainsail doesn't have any more sail area or better shape than an old shelf-footed mainsail, and isn't any faster.
Thus, what Jeff was saying, and I agree, is that, if you have a mainsail in good condition with an attached shelf foot, it can be expected to perform about as well overall as a new loose-footed mainsail made of the same fabric. However, a loose-footed mainsail is better than the attached-footed mainsails that used to be provided as standard, original equipment on most sailboats, because it has a broader range of adjustability, which is helpful in light air.
Jeff H was comparing an attached, shelf-footed mainsail
with a modern, loose-footed mainsail. He wasn't saying that the old, standard-equipment
attached footed mainsail is the equal of a modern, loose-footed mainsail.