His ultimate has to relatively low numbers due to its global nature. I'm an avid GP fan, and I don't have a problem with its current power. It's not going to annihilate someone's health bar on its own, but dropping it on another lane can easily turn around a fight.
Its potential AoE damage is actually very high; while it's single target damage got largely nerfed, it hits everyone in the radius every time now, making it deal far more holistic damage when it's dropped on, say, a team doing dragon or baron. The old ult dropped 25 cannon balls randomly; the new ult can theoretically land 35 cannon balls. If the entire enemy team stays in the radius for the full duration, the ability will deal a total of 5775 damage pre-mitigation. Obviously, that's never going to happen in a real game. But it's not hard to imagine hitting a total of 10-20 cannon balls if you position it really well in a team fight in jungle, or over baron pit--that's still 1650-3300 damage.
It's easily one of the most versatile skills in the game; it can be used to force a team off baron, help your team disengage or engage, finish off a low health target, turn around an early fight in another lane, cut off a retreating champion's escape, etc. etc. It pays for this versatility by lacking the raw, focused power of most other ults. I think that's a fair trade-off.
The E change OP suggested definitely needs to be made, though. Raise morale is a very strong ability, but it's incredibly clunky. In fast, early game trades, it's often counterproductive to even use it, since the cast time can cost you an auto attack. This change is a no-brainer as far as I'm concerned, and it should have been made a long time ago. I doubt it will get changed before the rework goes through, unfortunately. I was honestly surprised Riot touched GP at all before the rework.
EDIT: All this being said, I wouldn't mind seeing GP's ult get tweaked a bit when the rework rolls around. I think it would be cool if the cannon barrage intensified over time, dealing more and more damage per second and slowing more and more as it reaches the end. Slow and damage would be low early on, but would become a major threat to targets staying in it through the duration. This would reward GP for good ult placement, while also adding a bit of counterplay (get out of it early and it does very little).
