Do some research and you'll quickly find out that the V2 is the weakest of mechbox designs and the V3 is quite strong. There's no debate about that anywhere. I'd be interested in seeing the posts you're referring to saying the V2 was better.
As for their performance, I'm sure you're asking about their muzzle velocity. All TM guns perform more or less the same, shooting 260-280-ish fps, depending on the model and barrel length.