Hi All,
If I change from a 12-27 to a 12-23 Ultegra cassette, do I need to consider adjusting the chain length. Or is this change so small that this is not necessary?

You may have to remove a link at the most. After you change cassettes, shift to the large chainring and 23 rear cog. Leave just enough links so that it stays in gear. You may not have to do any adjusting at all.

If your chain is the "right" length now, then you shouldn't need to bother. I mean you already use a 21 tooth (or there abouts) now without problems, it just so happens that this will is where gears finish with the new cluster.

The biggest issue with chain length, IMHO, and which I trashed some gear 'discovering', is making sure the chain is short enough on the 12 tooth when in the big chain ring....the Shimano rear deraileur pamphlet that comes with the deraileur says the main wheel axle and both jockey wheel axles should form a straight line perpendicular to the ground. That's a reasonable amount of tension, and I think a lot of people don't have that much. Shifts are sharper and chains are thrown less frequently when the chain is that little shorter and under the proper tension by the deraileur.

IMHO, checking this tension in your hardest gear (12*53) is more important than worrying about how 'extended' or stretched the deraileur looks in your smaller gears when the chain is a little shorter.

OK, thanks a lot. I'll be sure to check the tension as well. Since I'll keep a 12t cog still, that shouldn't change.

You do not need to change your chain length. Because your staying with a 12 cog there wont be any extra slack.

I decided to give myself one last ride before deciding on getting the 12-23.

I did a 40mi hilly ride home from work just to see how things went on the 21t and 24t cog on my 12-27 set up. Fact is, I like doing hills, but like the fact I can fall back onthe 24t cog at the end of a grueling ride---the 16t cog is not worth it.

So, again, I may get the Ultegra 12-25---or---sigh---maybe consider getting the Tiagra 13-25 which has the 16t cog. Any issues with this fitting on my Ultegra setup with SRAM chain?

Sorry for being fickle and annoying for overthinking this...

You can get 13-25 in 105 or Ultegra too. Not every retailer carries them but they are definitely available so you may have to search them out or special order from an LBS. Not sure if available in D/A.