In a straight line up to 70 ish, there's nothing in it. Over that and the 645 will start to pull away, of that I'm sure. In the corners it's a different story, the extra weight shows up there and it's definitely not as nimble. On the longer corners and when provoking the rear it's actually more stable due to the longer wheelbase, but it's definitely a cruiser rather than a sports car!
That said, now I've done the suspension it's much better, and I'd argue better than a stock 350 up to a point. It struggles to get the power down as easy on corner exit as no slippy diff and I'm running relatively skinny rear tyres (on purpose), and with so much torque down low it's very easy to get the back end sliding. Which isn't always a bad thing...