HI,
thanks all. I'm going to put the badges down by the door and back wheel like VALKRYDERGUY and others have done. If nothing else, it maintains a level of consistency across a number of cars I have seen with them on. If Karmann were going to badge the cars, they would be in the same place across the range.

I'm not going to put the decals under the spoiler. Who's going to see that at speeds of under 602mph?!? I'm not in the habit of parking with it up (I feel it would invite the chavs to do damage). I'm planning on putting them on the back windows. I've seen pictures of them there, and I think they look quite good and discrete. The car being a dark grey would not show them up anywhere else. Discrete being the key word here - I'm not a shouty "look at my car - it's got a big engine" person - but I think it's nice to hint at what's there. The Karmann badges will do this well.

I'll do this when I have taken the cover off. At the moment, the sun is baking down, and the cover stays on until I need the car. Which will be tomorrow in readiness for a weekend trip.

Thanks all for the suggestions!