supposing the alignment is 'perfect' in gear 6; the biggest error is then in gear 2 and gear 10, and is nearly half a millimetre. [In gear 1 and gear 11 the mech position is defined by the mech limit screws, not the indexing per se.] Weirdly because of the effect of wear on the system, and the kinds of angle that the chain runs at, if the rear mech and/or chain is slightly worn, the shifting might be fractionally better on a shimano cassette than a campag one, even though it is 'wrong'. It does have to be more precisely adjusted though; basically you have used up all the inbuilt tolerance in the system before you have started.
cheers