I've just upgraded to KDE 4.13.2 - I am going with ubuntu LTS releases, and even though it went smoothly, there are a copule of cosmetic annoyances in the task manager I cannot fix: icons of miminized windows are grayed and desktop number and activity reference ase displayed in the tooltips. How I can disable it?

I'm afraid these are integrated behaviours of the Task Manager and cannot be changed. You can disable the tooltips if you wish however.
Otherwise you'll need to use an alternative task manager applet to get the behaviour desired.