I love Mao and would love to see the change. I think Mao would benefit significantly from OP's proposed passive changes without making him OP. Perhaps adjusting mana costs downward slightly (similar to recent MF changes) would also work. this would let him seriously compete with the other junglers that seem to be able to infinitely spam their abilities.
In terms of some other comments, his late game is not truly horrendous, but this is due to perspective and team comp. Think about it; late game, Maokai is basically a mana hungry Thresh 1.0:
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Instead of Death sentence (skillshot) where Thresh can hook (and stun) someone for 1.5 seconds, then, if safe, pull himself in, Maokai has Twisted Advance (non-skillshot) where he roots a champion for 1-2 seconds and flies to them; there is no going back, so if they flash, Mao's following (pros and cons there). Tradeoff skillshot for impact.
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Instead of Flay, which allows Thresh to shift champions on both ides of him slightly in one direction, Maokai has Arcane Smash which shockwaves forward (granting a slow effect) and has a 360 degree knock back on anyone within 100 range. Again a tradeoff, skillshot for impact.
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Dark Passage vs Sapling toss are probably the least similar but they still bear resemblance. Both are "tosses" and grant vision, but lantern provides protection and escape (massive utility) while sapling does only damage, and only IFF mao hits it, and the enemy does not outrun the saplig chase (which becomes easier late game).
Perhaps a slow (scaling by rank) on the sapling landing on or near a champion would balance this utility a bit. This would make Mao very dangerous early though, land sapling (and trigger slow) run in and TA, then knock them back with Q...
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For Ultimates, again both are similar. Area of effect zone providing team protection. Thresh sets up walls which deal damage and provide some CC when broken (and keep enemies out/in/away from squishies), while Maokai drops Maelstrom which reduces damage to all allies in its area by 20% (but chugs mana to keep it up) and then deals damage when the channel is ended.
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Passives both provide a bit of sustainability in combat, Thresh gets extra armor and Maokai heals.
Thing is, Thresh hooks provide a stun allowing other champs to kill the target while Thresh can shoose to hang back (and even allow assassin to escape with lantern), whereas Maokai must commit himself (and therefore the team) to diving in.
Thresh's lantern is also has massive utility all game in the form of assisting escapes (over walls etc.) with lantern.
Maokai's Ultimate is actually quite powerful, 20% damage reduction is very significant.
As noted however, unless very fed, Mao doesn't bring the damage...but consider:
Mao usually comes out of the jungle whereas Thresh comes out of support lane; Support are not expected to do high damage but Junglers are. This leads to the general feeling that Mao is "worthless in the late game. Since his team has to make up for the lack of damage." However, this is really just a team comp situation; with a high damage support, this would not be the case.
I strongly recommend a buff in the champion maokai. Mao is a splendid jg champion the only reason why maokai is not very popular is because he is so mana thirsty.... My only suggestion to enable maokai to be viable pick should be to upgrade (buff) his passive. Adding a mana recover with his health recover on passive should be enough in order for maokai to have more sustain mana and not being extremely upgraded. 
