Hey mate, it really depend on what the conditions are like where your surfing. If you have smaller waves with a bit of chop and not so clean, I'd say just stick with your 7'8", It is perfect for those conditions. If the swell gets a big bigger, then you will probably want to move onto a board with a narrower outline... the 9'0" hokua.
You could even take a look at the fanatic 9'2" ProWave, it has 135Litres, which is 10 more than the hokua, and it is 30" wide as opposed to the 28'3/4" of the Hokua. So it is going to be a much more stable board for you, but still have enough performance.
Marcel