11sp Campagnolo, Shimano and SRAM road cassettes do not have the same sprocket spacing, it's just that the difference is so slight that it makes no odds in normal use.
- Campagnolo 3.76mm
- Shimano 3.69mm
- SRAM 3.72mm
So if you index properly you could be about a third of a millimetre out at the extremes of the cassette.
But why would you do it? If you're going to splash out on 11sp, wouldn't you want all the components to match for maximum compatibility? The only application I can think of is if you wanted to run a wide range Shimano or SRAM cassette with a long cage Campagnolo mech and a double chainset, instead of using a triple.
Maybe if you needed a rear wheel from neutral service...