Hi Timo,
A subject close to my heart as you can imagine.
From the examples I have handled and have in my collection, I can tell you the earlier specimens DO have an 'Airfoil' or 'Convex' profile. The chakrum itself has an almost very narrow 'diamond' section profile, but a close inspection does reveal a slight 'aerfoil'.
In examples which I believe are later, the 'Airfoil' is more pronounced. I cant tell you which is better, I have never wanted to throw old chakrum around, although I cannot imagine it would damage them if thrown in open ground. Perhaps I will give it a go one day!
The British historian Mike Loades made a documentary under the series 'Weapons Masters' which some of you may have already seen. A very interesting and useful introduction to the Chakrum and aerodynamics of the object, I think it's a definite watch for you Timo.
Here is the link, although others in the States have told me it is not viewable there, perhaps it will work this time. If you cannot view it, let me know and I will see if there is another way to access the programme.
https://www.youtube.com/watch?v=sev9LmC_d_4
Good luck in your quest, and if you have any other questions feel free to ask.
Sincerely,
Runjeet