I would like a slighly longer wheelbase in that trueflite stock.
Hard to talk about flex and stiffness as I dont own a mcmillan f class. But I think flex and bounce in it would all be relative to the action and barrel you put in it. Brett B and Shaun both have rem actions and HV profile barrels so in essence not really heavy. Seddo sounds in a simlar situation. Where if you put a barnard with a 1.25 straight for 32 inch barrel with a heavy nightforce scope then your going to get more flex I would think.
My barrel block stock is made from hardwood laminate (structural stuff) and sealed and painted. I made it slightly longer than it was originally designed as I wanted as much support as possible. Most of the weight is at the front but I still dont think I get bad flex. We did think about drilling some steel or carbon rod up it, but it just hasnt needed it.