I think that when it’s pointed out (apologies for doing so!), it’s something you probably notice every time you walk up to the car.
Please post up a picture once you’ve had it done.
Edit; the link below to a discussion topic over on golfgtiforum.co.uk shows what the decal extended along the rear door looks like on a Clubsport. On the Golf, the rear door decal follows the door’s lower crease line;
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The crease line along the bottom section of the rear door kicks up on the
Cupra Leon whereas on the Golf it doesn’t - IMHO following the rear door crease line on your car along the entire rear door might be a bit too much, so a slight extension into the rear door as you’ve suggested would probably look better. Maybe the guys at Xtreme have a computer software package (they probably do) so they can show you different design options on screen so you can choose the one that works best for you?