Hi Sportse, Like you I am generally pleased with the Yokohama tyres and have just checked my tyre depths.
I find that based of 11000 miles, the wear is fairly even, with the rears a being 6.2-6.0mm and the fronts 6.0-5.7mm, which I guess is commensurate with yours, given the difference in mileages. The tyres on my previous Jazz cars were Dunlop and were far worse than this.
I don't however agree with your premise of the smooth drive, promoting a good mileage.
The Mk4 power, particularly torque is significantly more than the previous Jazz, which will improve acceleration and the general 'get up and go' of this model, and I'm sorry to say, that human nature being what it is, if the power is there, then you will use it, at some point.