It's important to remember that the sail is only going to twist when there's enough wind to make it do so - so that the amount of camber produced is variable. It's certainly there, and can be visualised by stretching a thin elastic cord from the heel of the yard horizontally to the aft end of the middle batten.

What I find is that when I have two or more reefs down, the upper half of the sail is twisting enough to produce all the camber that could be wanted, and there isn't any need to add any camber into the top three panels. In fact, I now wish that I hadn't put in so much, and when I next have to take the sail off, I may well reduce it. Fantail's sail has no camber in the top three panels.

However, under full sail in lighter winds, this effect is not present to any great degree. This means that if you want light airs performance to windward, some camber in the lower three or four panels is desirable. The battens are quite close together in this planform, and so the amount of camber that can conveniently be added is limited to 6%, in my opinion, which is enough for all purposes except inshore racing.

Having said that, if you made this sail almost flat, with just some rounding on the head and some minimal rounding on each seam between pairs of panels, it would perform perfectly satisfactorily as an easily handled cruising sail.

This sailplan certainly does not need HK parrels. Nor, other people are now finding, do other cambered panel sails - so long as careful attention is paid to the form and arrangement of the yard hauling parrel and the throat hauling parrel.  Paul Fay has recently written here about getting his high aspect ratio schooner sails to set well without HK parrels. In the topic on Tystie's new sail, I wrote: "I've changed the luff/throat hauling parrel so that it now starts at the batten 2 from the top, goes around the mast to a block on the yard, back around the mast to batten 3 from the top, around the mast to batten 4 from the top, and to the deck. I no longer have a second luff hauling parrel. I don't get the sail perfectly crease-free 100% of the time, but I get it good enough for me to look at without distressing myself."

If you want to add camber to the panels, you will have to decide on the method you want to use, and then layout the extra rounding on the panel edges, or draw an additional shelf, accordingly.