Having had to remove the rear spoiler from my C5 twice in the last year (high-level brake light then washer tube) I'm amazed that all that trim has to come off to access two of the four screws that hold the spoiler in place.
So the last time I did it I made a couple of holes in the interior trim to make it much quicker in the future. A couple of blanking grommets cover the holes and are hardly noticeable.
I thought I'd share what I've done so here you go :

(Of course, now I've made these holes I'm absolutely guaranteed not to have to remove the spoiler ever again.)