Even with the stiffest of arrows, you WILL get flex when you shoot the arrow. You need SOME give in the arrow to allow it to recover after it leaves the string. Too stiff of an arrow, WILL flex on the shot, but not enough after the shot to get back to EXACT center. Target shooters can get away with a lot stiffer spine as there is no canard on the front of the arrow (blades). For broadheads, this would make a very unforgiving set-up.
With a string loop, and a drop away rest, the bow is more forgiving to start with....allowing you to shoot a stiffer spine with good results. But even then, a properly spined arrow will give you a more consistant result. I have tried going up just 1 spine (carbon express...I shoot 200s, tried 300s) and found that while I can make the 300s shoot well, the 200s STILL grouped better...even on shots I thought were "bad".