with a ultegra 9-speed 12-27, would it be possible to switch the 17/19 cluster with a 16/17 cluster from another ultegra 9-speed cassette...and still preserve the hyperglide indexing (still shift well)?



the jump from the 17t to the 21t would be huge, but I'd love to get that 16t in there. I dont mind big jumps toward the end of the cassette and I think I would prefer it over the jump from the 15t to the 17t. and a jump of four teeth is not uncommon -- it's found on some mtn cassettes, as far as I know.

thoughts?

OK, I had been absent after forgetting my pswd, back now :-)

The deal with any mixing of Hyperglide-type cassettes has a lot to do with whether you'll be trying to shift while pedaling hard out of the saddle.
That's largely why HG was invented, for shifting with handlebar-mounted index shifters. It was a safety consideration.

Any switching of sprockets that doesn't follow the charted patterns shown will enable slippage during shifting. This is primarily related to shifting while riding out of the saddle, but is still somewhat noticeable when shifting while seated.

I am assuming you understang the intent of the chart's grouping, those pairs/sets of sprockets shown in one box are the same part number, showing that you can't build a perfect cassette that's not offered. All combinations that might work are listed in the chart, all six but only six.

Even the two adjacent 12-13-14-15t clusters should be shown grouped, but since they're not riveted together they aren't.
Note also there are two different versions of that 12-13-14-15t cluster.

????
What 12-13-14-15 clusters? Those are all individuals. What do you mean different versions?

They're not riveted, but the 4 sprockets have the same group letter i.d., and there are 2 different groups ("versions") for different size cassettes. These days, the letter codes are typically 2-digit, such as "aq", "ar", etc. Some cogs carry several different codes (such as the two I just mentioned), as they can be used (correctly) with more than one set of cogs.

Ok, I have several with XE (or XF) and XC. I've mixed a lot of 9-speed Shimano cogs, Ultegra, D-A, XTR, and others with no problems. They seem to shift just as well mixed as when in a new cassette right out of the box. My favorite 9-speed Shimano cassette is a 13-26 with Ultegra, D-A, and XTR cogs, 13-14-15-16-17-[18-20]-[23-26], very light and close ratio.