Putting a 8-10-inch taper on a cedar shaft will reduce the weight 30-40 grains depending on the shaft and amount of taper, i.e., from 23/64 or 11/32 to 5/16.
That will increase the FOC by 1-2% depending on shaft length and point weight.
Such a taper has a negligible effect on spine, reducing it from from 1-3#.
It does very little to increase feather clearance. For example, 11/32 to 5/16 is a reduction of 1/32-inch in diameter, or 1/64-inch in radius. The height differences in machine cut and hand chopped feathers are greater than that.
To the extent the nock is smaller, it would increase finger pinch by causing the fingers to be squeezed even more closely together, but again the difference is so slight as to be unnoticeable.
Tapered shafts do recover slightly quicker than untapered shafts.
That's about it. Not a big difference between parallel and tapered shafts, but several little things that potentially work in favor of the tapered shaft. I've shot both for nearly 50 years. I prefer the tapered.