I also upgraded from a 2011 p to a 2013 p+ less than three months ago. The electric power steering is definitely lighter than the previous hydraulic system, but I think that what you are experiencing is not normal but actually a common "issue" of the 8.5 models.
When working correctly, the electric steering should firm up in a progressive manner while increasing speed. Unfortunately, some people are experiencing the opposite when driving at highway speeds. There are several threads discussing this topic, but this seems to be the longest one:
http://www.audizine.com/forum/showth...ng-Issues-2013
According to this thread there are two main issues with the new electric steering.
The first is the constant "notchy" feeling when turning the steering wheel. As you stated, you don't have this issue.
The second problem is known as the "slip", which among other things results in a very vague steering feel, even at highway speeds. Many people report having troubles keeping the car straight. This could be your problem.
Audi is aware of the issue and is supposed to be working on a software fix.
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