Why can't Gangplank have a full mana refund on his Q instead of half?

His mana hunger is so low now compared to his previous version. I don't see the issue.

I think its so he doesn't get all this free gold and still has mana to harass you out of lane. You need to make a decision when laning. Harass or gold and SS.

Because of the goodies he gets from last-hitting sucks up the spell's power budget, so he only gets half, not a full refund.

He can. Use a barrel ( 0 mana) and kill at least 2 minions by Qing a barrel