Thanks for the feedback.

I am a bit surprised to read that your SON+eWerk couldn't keep up with the S5. Then I did some research and:

The S5 battery is rated at close to 11Wh. The maximum discharge rate yields 220 minutes, or roughly 3W.
The Garmin Touring is equipped with a 3.7Wh battery that is drained in, say, 8h, so that would be .5W.

Considerable difference indeed.

So, I guess, count me in the (somewhat frustrated) dedicated GPS camp...