Hi, I'm a currently diamond III top/jungle player that plays quite of bit of hecarim (when not banned) and I've been testing hecarim with the new Q changes and mana cost changes and my conclusion is that his laning and his jungling will become unviable if this changes comes to live. The Q damage to monster is actually a really good change and should stay, as it really improves his clear speed in early stages, where he generally struggles, while also helping his mid game clearing speed a lot, letting him clear faster and gank more without buffing his laning phase, which is the main reason he's a top pick right now.
Sadly, that's all theory. The big problem is, the mana cost. I tried to do a standard bot lane start with golems to red to blue, I was out of mana before I even finished my red. That pretty much forces you to do camp -> buff -> recall with no gold to buy anything except maybe a ward + some pots, but no smite upgrade or even boots due to you having no gold. Even if you do gromp -> blue, you will still have mana issues since the blue regen isn't enough to sustain the insane mana cost on his Q. On top of that, that totally destroys his mid game, espicially if you're forced to give blues away. In a situation where you have your cinderhulk or warrior + boots, you still have insane mana problems, which means you can barely clear the jungle AND gank, where you're supposed to be at your strongest. You are forced to take 2nd, if not also 3rd blue to compensate for that problem, which generally leads to your mid lane losing a lot of pressure.
That being said, afther you finish frozen heart, you don't really feel anymore mana problems, but that's not until 20-25 minutes into the game, where the game is most likely decided, most likely in a defeat since you couldn't affect the map with your starving mana cost in the early/mid game. I understand that you guys want to nerf his laning phase without killing his jungling, but the mana cost change would have to be a bit less extreme. I would suggest trying to make his Q mana cost be:
26/29/32/35/38 instead of being 32/35/38/41/44 on the PBE and 20/23/25/29/32 on live servers. This would hurt his laning phase a lot, but not hurt jungling too much since you will actually have mana to clear your jungle and have pressure around the map.