DEPENDS HOW FAR YOU INTEND TO CARRY THAT EXTRA FUEL:
ie: if you intend to use it on departure delays, not much of a penalty. If you intend the extra fuel to be available for holding at the end of a 14-hr flight, big penalty.
From memory, on the 747-400 if we carried 1000kg over from HKG to LAX, it would require 600-ish for en-route burn to carry it. Therefore, you would have to load on an extra 1600kg to arrive with the extra 1000. It was all printed at the bottom of the plan for the crews' perusal during flt planning.
I'm sure the Cessna has a similar calculation...en-route burn-off v. fuel on arrival...