I just did some research and concluded that instead of buying velocity stacks, buy 2 of the stock OEM short ones and make all of the stacks short.
Not sure about your bike but my stacks come with shorts ones on the outside (1 & 4), and the two inner stacks are long (2 & 3). So through my research. For about $20 bucks you can just buy two of the shorter ones from an OEM parts distributor and turn stacks 2 & 3 into shorties. Supposed to do the same thing as velocity stacks and save you a shit load of money. A guy on another forum reported a 4 mph difference on quarter mile. Supposed to move some horsepower to the top end and take some away from the low end but that's fine for 600s that are best are high RPM.