Just a couple of thoughts. The fully rotated mast will create a lot of drag and is normally used to increase power from the rig. The air trying to get around your mast could be the cause of the sail appearing to luff. Try less than normal rotation in strong winds. You say your sail seemed flat, where? Did it flatten off at the top under downhaul? Did the outhaul flatten the lower section? You can't use other boats as a yardstick of performance in those conditions, maybe they are struggling as much as you.

Try to get the sail to go flat and feather off at the top using the downhaul. If you can't get it to do this try a little more prebend in the rig and maybe try using stiffer upper battens. You still want to get some drive from the rig but it is better if it is lower down with the top section feathering in strong/gusty conditions. If you get this setup right you should absolutely fly on the shadow in those conditions.

Aside from the mast rotation it is possible you might have been luffing the boat!

Cheshirecatman