It's irrelevant.
You don't need Kaby Lake. The Skylake chip in the 27" is perfectly adequate and the 21" would be fine with a bump to Skylake.
What actually needs updating is:
- The 27" needs updated AMD graphics.
- Both sizes need the 2 Thunderbolt 2 ports replacing with 2 or 4 USB-C/Thunderbolt 2 ports (and, preferably, the other ports left alone).
If new Kaby Lake processors are available then hooray, but Kaby Lake alone is not worth an upgrade.
If you're not fussed about TB3/USB-C then, frankly, there's nothing wrong with the current 27". However, if Apple are serious about USB-C/TB3 being the future, they need to get a serious move on with rolling it out across the iMac, Mini and pro. Being able to connect an LG/Apple 5k display would be an advantage - although if Apple gave a wet slap about that they'd have ponied up the cash to wrap the new displays in a nice metal enclosure.