Highway mileage is great on the vettes but city mileage kinda sucks. I get 16/17 mixed. I doubt you'll get more than 19 on a C5Z. But yes, you will get 27+ staight highway. I think I'd take a C5Z over an LS2 C6. They're rarer now.

For a comfort DD I'd take a hard look at the IS. The LS430 is good but supremely BORING, forget it. G35s are a better choice but still kinda boring without mods.

Early cobras had issues but from what I know, late (03/04s) are good. The only downside is gas mileage and the douchebag title that comes with it lol. I like em though - have a mean exhaust note!