My longest range rifles also happen to be my lowest velocity ones. My Real Sword Type 56 shoots 300-310 and it has a sick, stupid effective range, and even stupider range (I lose sight of the BBs before they hit the ground). Next is my custom C8 at 325 fps. My C7 is about the same as my C8, and it shoots 380.
If you're going to upgrade a Kraken, upgrade the internals for reliability and consistency and change the hopup rubber. That'll be a much better upgrade than a spring. And as already mentioned by someone else, invest in a good quality battery and charger. And always use good quality BBs.