From past experience, it usually takes about 5,000 miles for the engine to loosen up a bit. This is where I usually notice gas mileage to be increasing as well.
It takes time to learn and get a feel for the car. Then minor changes are easier to feel.
I'm still breaking in the engine (only 900 miles on it) so it's tough to calculate gas mileage or performance.
I plan on doing an oil change @ 1,000 miles to dump the factory fill along with the break-in metallic shavings but will continue to use 87 to set a baseline MPG before trying out new...
If the 2014 "M3" has the same suspension specs as the previous model, it's only a matter of time before manufacturers test their coilovers from the previous gen, on the 2014 model, to confirm fitment and then announce it to the world.
It's better to buy coilovers that have been on the market...