It is actually not the Apple release cycles one should be concered about. As an example Chromium is on a continuous build cycle where each day they are around 5 to 10 builds made.
But there is a concern in another area. Webkit coders are generally protective about patches and new features and trying to get a patch approved in upstream is usually a very tiring process right now. Sometimes they demand a complete clean rewrite of code as they are worried that the patch might break something. Code review process also tend to be slower and absurb statements like this would make Safaris UI more complicated seems to be not uncommon. I believe at one point Chromium developers will start to mantain a trunk that is a bit differnt from upstream. Right now they wait untill it is approved with webkit.

One of the reasons why Google came out with a Browser is to support Webkit. A platform that they consider is crucial for Mobile Devices. By increasing the mindshare of webkit, Google is able to push the Mobile Web in a direction that they want. Safari made a nice dent in the Market share, and the iPhone went further. Nokia is running a browser based on Webkit. So trying to speed up adoption of this makes a lot of sense. Right now sales of mobile far outweigh the sales of Desktops, but the browsers that run on them still leave much room for improvement. This where webkit comes in and makes mobile web apps more appealing and powerful. The browser for Google is a medium for them to push Ads through. Trying to gain first hand knowledge of Browers, and of web application developments on these browers is importnat for Google. As an example, Gtalk for Gmail offers more features than the Desktop version (which now includes Video). Since this is the medium where they can push Ads on they are more focused on making that version a priority. I doubt that these features really came from the 20% time. Push the Web, understand the Web and Extend it. Kill the Desktop. That's their mantra.