I've never been a great fan of the Campy or IRD low gearing on their wide-range road cassettes, which have a 23 to 25 tooth transition on their 12-25 (Campy) and 12-28 (IRD) cogsets. That close ratio shift may be great for young folks or racers, but I want a little bigger ratio drop to get this fat old hind end up the mountain with a higher cadence.
This morning, just for grins, I combined the eight higher (12-23) cogs from the IRD cassette with the 26-29 low gear pair from my old Campy 13-29 cassette to get a 12-29 ten speed cassette with a 23 to 26 tooth transition and a 29 tooth bailout gear for the initial brutal grade at the bottom of Camp May Road. Initially, the Frankengears was not spaced properly at the splice but then I added an extra ~0.8 mm spacer to the existing spacer between the Campy and IRD fragments and now it shifts as well as either cogset.
The reason I don't just use the Campy 13-29 is that I sometimes spin out on the 13 cog using my compact 50/34 crankset. And yes, this does exceed the listed maximum capacity for the Chorus system (using a med. length Chorus rear derailleur) but you can fiddle with the chain length (take out a link or two) to keep the upper derailleur pulley off of the low gear cog. I'll post a picture if anyone else is interested.
I took this Frankenstein gearset out for an inaugural run up into the Jemez and up Camp May road and it worked great, using the 26 to climb the steeper parts of NM-4 and the 29 for the bottom of Camp May.
Wish my legs and lungs worked as well. But its nice to know that one can customize, mix, and match this stuff rather than being stuck with off the shelf components. It just takes a little tinkering.