The question that is hard to quantify is how much slop is in the system.

So, Campy 9 speed has a 4.55mm sprocket pitch.
A Shimano 8 speed has a 4.8mm sprocket pitch.

Over 7 spacers, that difference would amount to .25mm per sprocket, or 7x.25 = 1.75 mm. That is probably a bit much for a derailleur to take up.

However, if one sets the derailleur to the middle sprocket. Then each direction is off by about 0.9mm, and it may work.

There are changes in cable routing that might help.

If you look at the Shimano spacers, they are listed as 3mm for Shimano 8 speed. You need about 2.75mm spacers.

It might be possible to file or grind just a little off of each cassette spacer to improve the shifting, especially if they are plastic spacers.