Originally Posted by bluehills3149

I also found the 10 spd cassette with 9spd spacers to shift badly. Probably because the gap between the cogs is too great and the chain is not wide enough to catch the adjacent ramps.
My solution (again, Camp 10 spd shifters and Shim 11 spd rear der) was to modify the cable wrap pulley inside the shifter. I had a spare one as there was a replacement for this part (Campagnolo part EC-SR060) because the original shifted poorly so I was not afraid to hack mine. If you search they may be still available but this solution only works for those campy shifters using this pulley.
This is also the basis of the "growtac equal pulley" that is sometimes sold on ebay (and is shown in the linked youtube I posted earlier in this thread) but in their case they increase the size of the pulley to use a Shimano 10 spd rear der.
There is also some info here:
Shimergo or Will my x speed shifters work with a y speed mech and a z speed cassette?
I used a file to take 0.5mm off the radius of the pulley (which is made of a fibrous plastic) which I calculated would reduce the total cable pull by about 2mm which is what I needed. The pic shows (approx) the arc in red where I reduced the radius.
It now shifts perfect and you can see the der lines up with each gear exactly.