The pipe that connects to the Cat on the V4 exhaust is 70mm, it seems to be the same part used on the V3 (it's just the cans that connect after the split that are different. I'm not sure where all you have seen 60mm posted, I just remember it being listed on a website of a company that sold the V4, which you can take with a grain of salt. Obviously it was a mistake on their part.

And yes, the Q300 is the same exact system as the V4, it's just the official "model" number from Invidia.

 

so does anyone got a site with a good rep to buy from?

 

I've used several venders here at s2ki and they are all very good.
Bought my V3 from one of them so you might shop for price since several do carry the Invidia.

 

bought my v4 from hopupracing.com along with the test pipe and received the exhaust in about 4 days all the way from california to delaware