Total nonsense. Valve size has little to do with torque. Some day, all these wives tales will die off. I hope.
Some things are counter intuitive and exhaust valve size is one of them.
In total agreement ^^. I use 1.65 exhaust valves. Because normally the seat has to be cut and that means I don't have to have the seats replaced.
Camshaft will be the biggest factor given the usage of manifolds. Strokers with some real science behind them will exceed 550hp with extrude-honed manifolds (FAST class). You will give up power over headers but they won't ruin anything with the right cam. If you seriously think a 4" stroke engine will "be down on torque" because of valve size you have to go drive a few cars with them before you order the 3/4 race cam.