I've only been to 2 autocross days and it makes no difference if the top is
on or off, except for the helmet not banging on the top on the corners.
Think about the top on your C4, it was bolted fast. The top on the C6 is
held on with a plastic over center clamp in the back and corner pegs.  The
top on the C4 was structural and on the C6 it is an umbrella/window and
weighs about 1/2 the C5's top, which was less then the weight of the C4 top.
The first time I jacked up the right front corner of my C5 it also lifted
the rear tire off the ground and the C6 is a bit stiffer. Other than the
wind buffeting it is a sound and stable ride. The convertible is a very
firm, quite, and stable ride also but I think I'd skip the power top option.

You need to drive the C6, just a tad better than the C5 and light years
ahead of the C4 frame.

The C6 is the most stable car I have ever driven. Period, paragraph.

My C6 convertible is also one of the most rigid cars I've ever driven.
That thing is *solid*, more so than any other convertible I've owned.
The only cars in my experience that rival it are a VW Beetle (a real
one) and a Lexus LS430 - both sedans.

The C6 convertible top is not structural, of course. It's a folding
cloth design, not a removable hardtop. There's no difference at all in
the car's feel between top up and top down aside from wind and sound.

For that matter I don't think the coupe's top is structural, either.

-- V

Thanks guys, great info. Now I know what to expect.

Mike