The spacing and braces on the convertible are different(so im told). As for the ground effects they are only compatible with certain tips due to the size. After making quite a few calls directly to companies I found out that flowmaster doesnt make an exhaust to directly fit a convertible with gfx. Corsa makes only one system. Borla has a few systems that fit but the tips are different for example, the atak system has a GFX specific system. Hope that helps the next person that comes across the same issue.
Lil' more info...
I dealt with "trying" to install a Flowmaster American Thunder on my vert when I first got it in Feb 11'- didn't work so well... actually, impossible, is more the proper term.
FM makes several axle-back kits now that fit verts. gfx and non, Magnaflow just released their competition series cat-back for the vert w gfx and most other companies are compatible as well... F.M. is really the only one dragging their feet still...
as for issues of GFX/non GFX, the tips are the major factor, but there's also a hanger issue... I ended up with Magnaflow Comp Cat-Back last summer and had to have a muffler shop form the hangers to fit the non-GFX system to my GM GFX, appearantly they wouldn't line up correctly.
The tips themselves are a whole other issue, many systems just cut the pipes inside the bezels using the bezels to appear as a tip themselves, while other companies like the MBPR (sp????) have the rectangular tips which sit outside
the bezel, this is important, as, I learned the hard way, when you start adding more HP and less air restriction, the fumes are hotter, and you can melt the rear diffuser if not placed properly... at the time, mine were flush with the bezels, looked great, but the melting and discoloration sucked.
I ended up fabricating a quad exhaust tip system with powder coated bezels... gotta be different.
Hope that helps!
One other thing, for what it's worth...
Due to the clearence restrictionsof the additional bracing on the vert, without going to after-market bracing, your not gonna exceed a 2.5" pipe... you may want to concider an axel-back system for 1/3 of the price, and swaping the "H-pipe" for an "X-pipe" for a few hundred dollars... save you $300-400 over buying a cat-back system in which your swapping 2.5" pipe for... well... 2.5" pipe. I didn't figure that out till it was too late.