I've done a 2-week trip through Northern Europe last summer, albeit in a diesel Mazda, not a coupe, but some general advice:
Re the deflectors, follow the general placement advice on the stickers, i.e. 2-3 o'clock placement (if my memory serves me right) I found they don't do that much anyway no matter how fitted (on my Mazda 6, not Coupe, which actually had specified instructions) but if you get pulled over, you can expect some leniency as you HAVE them on; nor did I find that the authorities were too bothered by a UK reg car - slap a big GB sticker on the boot and feel like Moses, as everyone tries to give you way before you do something stupid
Also, keep in mind that some parts of the Autobahns do in fact have speed limits, and some bits are 'Smart Motorway'. Also, as far as Germany is concerned, take off 3-5 off your planned MPG if you're going on a dual-carriageway style autobahn, as slow lane is jam-packed with lorries and the fast lane is full of people doing 120 MPH -- and we all know which lane we'd all rather be in.
Avoid the Duisburg-Dusseldorf-Cologne area during peak times - the queues are amazing.
And above all, drive respectfully regarding other drivers, as the Germans
will ring up the police and report a 'dangerously maneuvering vehicle'.
Viel Glück