I've seen the V8 Vantage Coupe go around Road America, and it was very fast, and gave very little up to the V8 M3 guys who were also out on track (could be he was a better driver). In convertible forms, if I had the means I wouldn't think twice about upgrading from E93 M3 to Vantage, and leaving it in loud sport exhaust mode all day long
No comparison as it's would be just as fast for any driving anywhere near legal limits of public roads. And that's all that matters as your not getting the vert AM to track, so who cars what it does off public roads....