(a) I've only used ultegra, which works very well and is quite a bit cheaper than DA. If you are obsessed about shaving weight and have money to burn, go DA. Otherwise, ultegra will be absolutely fine.
(b) When you switch cassette sizes, you sometimes have to do some fine-tuning of the rear derailleur. This takes about 5 minutes. If your chain is cut very short, you may have some issues going from a 23 to a 27 in the back. (Quick way to check on this. Go big gear in front, 12 in the back. If the jockey wheels on your rear derailleur are vertical or pointing to the back of the bike, you'll likely have a long enough chain to use a 27. If they are pointing forwards, you might end up on the short side).

You're doing IMOO next year, right? I'd recommend at least a 25 on the back for that unless you consider yourself very strong on the hills. If you have a compact front then 11-23 should be OK.

The adjustment is really very simple--just putting the bike on a stand and using the barrel adjuster to get good shifts. When I put on race wheels, I usually take the opportunity to clean the chain too, so it's not much extra work. Also, sometimes it seems that even when I am using the same size cassette on my race and training wheels I need to do some fine-tuning, so going with 11-23 on each might not stop you from having to do the adjustment anyway.

For the chain, you can cut a chain so that it works fine with both an 11-23 and a 12-27. If your current one isn't cut that way, then you may need a new one. But you definitely only need one.

SRAM does a wider range, like 11-26 or 11-28. Might be worth looking into. I've got an Ultegra 11-23 and 12-27. I live in Boulder, so I run the 12-27 except for relatively flat races. It takes 5-10 minutes to change cassettes.

Theoretically SRAM and Shimano cassettes are interchangeable. My experience was that everything was fine with 9-speed, but with my 10sp setup (DA derailleurs, Ultegra chain with Wipperman quicklink, FSA crank) the SRAM cassette had one or two gears just would not stop chattering. It didn't matter how much I adjusted the derailleur, though sometimes the chattering would switch to a different gear. I think at one point I even tried a SRAM chain and that didn't solve the issue. It was a bit of a problem since the loud gear was one I use a lot (like the 15 or 16). I switched to a Shimano cassette and the noise went away. Your mileage may vary.

Cool. I'll do that (12x27) if I need a different chain, so be it. I need to learn how to maintain my bike anyhow, I've been a rockstar for too long.

Chain tools, by the way, are an item where it pays to splurge a bit. Learn from my mistake and buy a good one. There's nothing worse than gouging the palm of your hand trying to remove a pin using a cheap chain tool. The small ones just cannot provide the torque of the bigger shop-quality tools.

And while we are on the topic of tools, if you end up getting more than one cassette you will need a "chain whip" to change cassettes.

http://www.parktool.com/repair/readhowto.asp?id=48

Thanks for the tip.