Is it possible to get a banner module between intro and full article text in article view? With a plugin or a hack of core-files. load module inside article is not an option since I want the articles to be as clean as possible in case of changes in layout later.

:-)

You need to insert a module in your item.php between the item intro and full text.
The code is very well commented and you can easily find these blocks.

Remember not to edit core files but override them instead. This tutorial will help you:
getk2.org/documentation/tutorials/174-templating-with-k2-and-the-concepts-of-sub-templates

This post will help with the code required to display a module:
getk2.org/community/New-to-K2-Ask-here-first/179921-Add-a-Module-position-to-K2-Template-do-it-better#179921

Thank you :-) !