What cassette to use w/ race wheels

I could be wrong, but you probably just need to adjust your derailleur to eliminate the ghost shifting because the cassette spacing could be ever so slighly off. I'm not overly familiar with zipp's but I wouldn't think you HAVE to use a specific cassette.

I'm far from an expert on this, but just switched cassettes after a not so fun experience doing a tri in the mountains with my original 11-23.

1.) Swap cassettes based on terrain. Flat = 11-23 Hilly = 12-25. You'll need a couple of tools, but they're not expensive and the task itself is very simple. Now I just pack both and pick the casette after looking at the course.

2.) Not a huge difference really between the two casettes listed. But you will probably need to adjust the rear derailleur after swapping the cassette. I went from 11-23 to 12-27 and had to spend a couple of minutes dialing thigs in again...

How old was your chain?  A worn chain with a new cassette may not mesh well causing noise or shifting problems.  If readjusting the RD (rear derailleur) does not fix things, try a new chain.

Your choice of cassette should depend on the terrain, not tri distance.  Going with a 12-27 for a course with steep climbs can be a big advantage.