I guess that is what I am saying. The 2x4x8s I have been getting recently don't match what I have used in the past closely enough to trust my experience. they do not hold fasteners as well, split way more easily and warp a great deal more. As for knots is is obvious that some pieces have come from extremely small trees.You definitely can get "exact" stress load and support requirements using the sagulator online calculator.
I did so when building all of my 180 stands. SYP is not an option there. I can't get syp on the west coast. Our 2x4 are mostly Douglas Fir.
As you elude to, of course there are differences. Pine vs spruce vs syp all have slightly different qualities. One knot in any of them could change the integrity hugely. My above ascertation that 2x4 is enough, In this instance is soley based in experience building.
A quick once over of HD seemed to show the better stuff is being used for 12 foot and longer boards. The 2x6s I just used to build my 180 stand also seemed the same as I always got here. I told my wife I would hate to have a house framed with the stuff the 2x4 are made of.