who says roku could see into the future, aang had barely grasped water bending... by the time he faced the firelord he had basically mastered water/earth and since the comet was there his fire was pretty top notch too.
you've gotta realize, that even w/o the avatar state everyone (besides aang and his gaang) including the firelord thought that in a normal battle without the comet the avatar would win... so considering that and how ozai did beat aang before he entered the avatar state, the comet seemed to have made a difference, they prolly woulda been evenly matched if there was no comet. With the avatar state tho, there was no question who was gunna win.
Also realize that roku most likely didn't think aang would be able to control the avatar state by the time the comet came... (and for all you who don't think aang could control it...) all the past times he was in the avatar state for a few seconds really and just destroyed everything without thinking, while with the firelord he was actively aware and even stopped himself mid avatar state, not to mention he wasn't tired after the fact.