I'm confused about the 30 cog swap. Aren't the largest few cogs permanently attached to the spider? It is my [limited] understanding that only the smaller cogs are removable and interchangeable, at least on the more expensive cassettes.

As for the shift quality-- the ramps are engineered to provide optimal shifting to the neighboring cog given a specific number of teeth difference, but in reality, it's not noticeably different if that tooth jump varies from the original cassette. Harris Cyclery used to make their own custom cassettes for customers based on their own preferred ratios, and I think they even mentioned on their website that the shifting is negligibly effected.