I do think the OldFox is right
Esp. since that is a Bose system, they'd try and give you the smallest possible speakers.
A bit of googling shows that you most likely have 6.5" midbasses in front with 4" midrange drivers. No idea about the tweeters they should not be an issue -- 3.5" or lesser for sure. The rears are most likely 5.25". This is the link that seemed give some sensible info:
C5 and general audio FAQ - Corvette Forum
The TRUs I mentioned are amps -- the S44 is a 4-channel amp (to drive 2
pairs of speakers) and the S500 is a mono amp (to drive a subwoofer -- or more, depending on the voice-coil configs of the subs). I think the going rate for a TRU S44 is $799 or so (6spdcoupe at DIYMA is a TRU dealer) which means we need to strike that off.
Like sk456, I'd also recomment Image Dynamics subs with the speakers you're planning to use. Out there, you should get an ID12 D2V3 (12" sub with dual 2ohm voice-coils) quite easily. Try shopping at woofersetc.com for good deals. They are an authorised ID dealer. The ID sub I mentioned goes for $179.99 there.
You should probably look at a very good 4-channel amp and use 2 channels to drive the fronts and bridge the rear channels to drive the sub. The Rear speakers can be run off the HU (or you can ditch the rears altogether -- I don't think you really need them in a 2-seater). I do think you might have to up the budget by $100 or so to get an amp good enough to drive those components and a sub, though.