Campy 8s shifting 5mm on Shimano's 4.8mm spacing could work okay because it's overshifting. Could even work sweet, with the right sort of slop or mush in the system.
Not so much with an undershift. OP seems to be used to something petty sub-optimal. 2nd-gen Shimano 10s can be pretty crap out of the box, but any other Shimano system, and it'd have to be worn parts or excess friction or something. 8s should be quite precise, 9s a little less so and 10s a tad less still, although the actual shift of the chain from cog to cog gets nicer as the spacing narrows, if that makes any sense.