I do pretty much the same thing give or take 0.2-0.4 l/100km if my sound system is pumping alot of jouce is goes does a little, but nothing major. I love this car even coming from a tuned 420RWHP BMW 135i, I still love my CT
From my own experience, this seems a bit too much.
I'll express in km/L,
If I go at 100km(about 60m) I do about 4.8l/100km(49mpg)
If I go at 115km(about 71.5m) I do about 5.4l/100km(43.5mpg)
So I'd say 2 for 1 based on my experience on the highway