I'm posting from a ChromeBook pixel running at 2560 x 1700, and the UI looks great. The taskbar, UI, fonts, etc. are all scaled perfectly.
Have you tried running at 2560 x 1700 on the most up-to-date version of ChromeOS? Some of the issues you had may have been fixed in more recent Chrome releases.
Have you tried running at 2560 x 1700 on the most up-to-date version of ChromeOS? Some of the issues you had may have been fixed in more recent Chrome releases.