in my experience, it does effect spine, but only minimally, the extra mass adds weight to the rear and this stiffens the spine a wee bit.
i have changed back and forth between different lengths, and i am AR about Tune, i havent seen a difference between fletch lengths.
more of a difference going from 2 to 3 to 4 fletch, but thats due to the extra mass.
just my .02c
i used to bare shaft, and when i did i always tuned a tad weak, if anything.
But, now i only broadhead group tune. i find it easier, more consistent and easier on shafts