I just installed these same springs in my 05 convertible tonight and I ran into one additional snag when doing the rear springs.
The convertible has additional bracing under the rear axle that prevents the axle from dropping more than about 2 inches after you unhook the rear shocks. So, your choices are to remove the bracing (didn't look easy to me) or to try to get a spring compressor up in there to compress the rear springs just enough to pry them out of there.
Sucks because there is not much room up there so I had to manually tighten that spring compressor with a crescent wrench... Pain in the butt, but they're in now (3 hours later).
Anyway, just thought I'd let any of you convertible guys out there know what you're in for on the backend.