Rob-
Agree on the Pentex seeming to be the way to go. All my 4.9 sails so far have been Mylar but I have a Pentex (Goodall) main for my Flyer and a Mylar and the Pentex is a little "stretchier" so it can be cut fuller (more powerful) but still be depowered sufficiently. I believe at the recently concluded A cat Worlds most of the top boats were using Pentex (Ashby won by the way using his own Pentex sail).
Chuck- If you are going to custom order a sail then DO go to the trouble to do the mast bend measurements on your mast and provide to whichever sailmaker you choose to use. Also, you may want to see if there are others over here who would like a sail also and put in a "group" order- Often cuts costs for all involved. You will have to decide if you want a "pure" sloop main, a "pure" cat rig main or one that will do well either way AND whether you want it Taipan 4.9 class legal or F 16HP legal.

Kirt
Taipan #159, Flyer #185