If you really needed to go over 180 mph in a GM car, you'd use a Corvette. Used ones are surprisingly cheap and I'd venture to guess that all you would need to break 180mph in a C5 (OOB they do 175mph) would be a taller top gear and some engine electronics, although there are obviously lots of engine modifications available. For a little more than the 12k the parent post mentioned (I'm thinking 15k), you could have an expert bore the engine out and get an extra 130hp. For less than 40k you could be driving a real monster.

These sorts of mods are common enough among drag racers, except most of them aren't optimizing for top speed.