I don't know enough to comment on the stiff vs flexy mast thing.
but .... I have seen first hand how if your mast doesn't have a consistent bend, not good. If you can get your sails match the bend in your mast, that is a very good thing.
Yes, I do have a story..... In SF Bay, every other year, totally under the radar, there is an international invitational regatta with identical keelboats, just for the fun of sailing, no pros allowed. I went because they borrowed my boat and I delivered it to the SFYC in Tiburon. I had a beer and watched the top German team, absolutely torture my friend Dave's boat. They had six guys hanging off one of the shrouds while they sighted the old wooden mast. Christoph, the German skipper, told me "de boot, she is slow" and they were trying to determine why. How can that be? my buddy Dave takes perfect care of his boat, it sparkles and besides they are all identical. The Germans placed 12th in the practice race, and they were not happy, so while everyone was partying at the club, they took the mast off this really crappy old boat and swapped it with the really pretty mast that was on Dave's boat. They were much happier the next morning when they were busy tuning the boat. First race - 1st place, and I had a lot of questions for Christoph. They had one of the crew go up the mast and inspect everything on Dave's mast. There was a section of the mast the the wood was a slightly different color and they couldn't get an even bend so it was impossible to tune that mast. They brought two sets of sails from Germany, one for a stiff mast, the other for a flexy mast and they didn't like the set of either. The crappy old mast that they swapped in, it looked really bad, hadn't been varnished in years and had moss growing off of it, but they loved it, because they could tune it. When I caught up with Dave, I told him the story and he fessed up and said, yes, there was a repair done to his mast years back. He had a new mast made.
Here is a small article I wrote up years ago -
http://www.sfbayfolkboats.org/archives/ ... ER658.html