So I’ve gone round and round on these, and I’m of the opinion that the stiffness differences of all of the different interfaces just boil down mostly to BB shell width design in the frame. I’m riding a cross bike that has a PF30 BB that I’ve retrofitted a 386EVO crank using spacers for the extra width. I have a road bike thats setup with 386EVO originally, and I have two other older road bikes that run variations on threaded BBs. I went with 386 EVO on the two main bikes because thats the format of my Stages powermneter, so I can switch it back and forth with just a few minutes and a 5mm hex. A quick calibration before I roll out and I’m good.
The only bikes I can tell the difference on the bottom bracket stiffness is the older Cervelo thats primarily a trainer mule now, its got a narrow BSA and I can definitely sway that bad boy back and forth. As I say, I chalk that up to the width and stiffness of the frame around it. Park it next to my MJ or R3 road bike and the down tube is absolutely spindly compared to the wide, fat tubes on them. That being said, I have an older Pinarello Prince, and it is stiffer in the BB than any of them, and it uses an Italian threaded. It also beats the nether regions into submission like it was its job, so bad with good. I feel like I should mention that neither of my PF frames have creaked, but I KNOW I’m in the minority there.
Anyway, boil it down to, my opinion is that all the 30MM x 100+MM cranks are equally stiff in a good frame.