As much as I completely believe you guys about what he said, and I am not discounting his credentials, analysis of images simply says the wheel base is not 300mm shorter, I am 100% certain of it.
The overall bike maybe 300mm shorter, but there's no way in high hell they would release a Rocket 3 with the wheelbase of a Yamaha R6/Daytona 675.
I suppose I should add that it's a good thing they didn't go crazy reducing the wheelbase of the new rocket. While a slight decrease in wheelbase and a slight decrease in trail will improve handling going all the way down mid 50s inches would make the bike extremely unpleasant to ride and basically ruin any sort of stability. The dimensions they ended up with are very close to what I ended up with by lifting the back of the motorcycle almost 2 in inches and lowering the front about 1.2 inches as many have done with longer shocks and a 140/75. The new bike will have more trail than the aforementioned modified bikes do and a steeper fork angle, so it'll probably feel even lighter and simultaneously have more stability.
It's going to feel about 200 pounds lighter then the current bike when in stock form on the road.
EDIT: Added the 52.88" line to show just how far short the 600cc sportbike wheel base is in comparison and why its not even possible.