The Nexus 5 and Android 4.4 might be arriving very soon, but Google has already been working on updating all of its apps on the Play Store, a couple of days before the Kit Kat announcement. They've just updated all of these apps (Hangouts may still not be updated for everyone yet, but it should be soon, which means you can enjoy SMS support):
This isn't exactly the first time Google has done this, although perhaps they've never updated so many apps at once, and around the time of the new Android version launch. This may be because there could be a lot of new changes under the hood for Android 4.4, and Google needed to upgrade all of its apps to take advantage of those changes. That also means that unless you will get Android 4.4 soon, you might not be able to take advantage of those changes.
Since Google is going to get the Linux kernel 3.10, if they took advantage of the kernel's performance enhancements such as lower RAM usage, 64-bit support, faster storage, then some of these apps may be able to take advantage of those changes, too, although the 64-bit one won't happen until there are 64-bit ARM chips in Android devices. Right now, that's not the case, as 64-bit ARMv8 chips for Android won't arrive until next year.
I'm glad to see Chrome is on the list, too, and I'm hoping it will full advantage not just of Android 4.4 changes, but also of Qualcomm's Snapdragon 800 chip. Last year, Google launched the Nexus 4, but it didn't have a new version of Chrome on board, and Chrome was still behind the desktop version in terms of features and performance by a lot.
It created quite a controversy then, because reviewers like Anandtech used Chrome to benchmark the phone's performance. But since Chrome did so poorly, then everyone thought the phone itself wasn't very fast, at first, which of course was wrong, because the hardware was at least as capable as its main competitor at the time, the iPhone 5, in both CPU and GPU performance.
I'm hoping Google has learned its lesson, and this time it will perform as well as possible on the Nexus 5, otherwise we might be looking at some disappointing benchmarks again. That being said, we really should not put much faith in benchmarks anymore. Sunspider especially is a much more simplistic benchmark than something like Octane or Kraken, and Google has stopped caring about it a long time ago, so expect Chrome/Nexus 5 to perform relatively poorly in Sunspider again, but do very well in Octane, Kraken, and other such more advanced browser benchmarks.