I'll merge that pull request now. There are already a lot of improvements in it.
We now have 1015 tests for the useragent parser (covering nearly all os and browsers that are detectable by now). The detections takes around 2.3 seconds, which means around 2,3 ms per detection.
We still need to improve some detections and we need to think about the browser & os families again as they are currently not very useful. Followups to this and other improvements will be discussed here: http://dev.piwik.org/trac/ticket/4215

Excellent work Stefan! I plan to put some work into this to place some extension points to the Device Detection and some settings. I think this will lead into the opposite direction regarding the optimisation efforts discussed here, but we need some tweaks to use the full potential of piwik and would take the downside of this. Just a few Examples:

• We would like to add a lot of specific device Information to distinguish devices.
  Small example: the current Version would detect "Dalvik/1.4.0 (Linux; U; Android 2.3.3; GT-I9100 Build/GINGERBREAD)" as "GT-I9100". But this device is well known as Galaxy S II and the Modell in our opinion should be "GT-I9100 (Galaxy S II)". I guess all this information isn't appreciatet by all piwik users (additional regexes), so my thought was to indroduce a folder where people who love more detailed information can place custom regexes for mobiles etc. which take precedence against the "normal" regexes.
• We have several sites where iPhone Apps connect to webservices using UserAgents like this: "Ourapp/2086 CFNetwork/672.0.8 Darwin/14.0.0". Where Ourapp is the appname and 2086 the build number. I would like to introduce a setting for the device detection to translate this into dynamically created Browsers so the Device Section has some insights on used app versions.

Any thoughts on this? Should i file this somewhere else since a closed pull request dosn't some the right place to discuss these additions.

The next step will be to combine UserSettings and DevicesDetection plugins and to enable it by default.
Maybe you could create a ticket for the changes you would like to have at http://dev.piwik.org.
If you plan to implement anything on your own, feel free to create a pull request.