As you have already mentioned, there is really no information about higher fishing skill bonuses. However, these might be some of the benefits:
Having a higher fishing skill might help you to catch rare fish more frequently. Some of the rare fish are still havent been found, so they might require a higher fishing skill to catch.
One thing some of us have noticed during the test was the more frequently caught sea serpents with legendary fishing skill. So, higher fishing skill might help you to catch more messages in bottle. Maybe, even more white pearls and delicate scales.
Most of the fishers want a higher fishing skill to get better orders from the fish mongers (together with a better reputation). I dont know if some of the rare fish baits, volumes of the dragonfish handbook and/or fishing powerscrolls can be obtained by these higher orders. It has been recently announced in uoherald that even the drop of 120 fishing powerscroll from the fish monger quests is currently being reviewed. I guess we will all have to wait and see to have a correct answer to your question.