The Dacron Calvert sail is priced pretty fairly at $588 and is definitely faster than Whirlwind. Here is the link:
https://store.catsailor.com/pc/viewPrd.asp?idproduct=30719&idcategory=257

As I said the one I have is full, so I max out the downhaul and sheet really hard, hooking up the sheet as far forward as I can go, and that flattens it out pretty well. In light air, just ease it off.
Rick