Lol, it seems you've covered it already.

That's very easy to abuse. Whales have a clear incentive to vote against the little guys and essentially rob them of their value, because it wipes away their rewards increasing the available pool for the whale.

I see a huge window for whale abuse here. The mechanics seems sound, but the psychology isn't.

imbecile

So.. A whale could use their 'bonus' downvote power to downvote anyone they don't like the sound of or who disagrees with them (perhaps even disagreeing on the future of Steem, for example). They would then be rewarded for silencing dissent AND increase the rewards available for themselves and their self upvoting in the process.. Win, Win.. for the whales. Lose, lose for anyone who disagrees with them.
I recall we talked through this a few months ago and we didn't reach an agreement - I seem to recall you acknowledged that a separate downvote pool has it's problems.. I still think that a separate downvote pool has HUGE problems and as others have pointed out - many of them are as much psychological as anything else. New users don't like diving into a pool where the food sources are already heavily controlled by a small number of 'fish'. If those fish are actually sharks and now have the ability to not only hoard the food, but actively remove it from others on a large scale - then very few people will want to be here. 
This community relies on those with the most stake making 'good' decisions for the community, but in reality, few agree on what that means and the typical approach seems to be "well, of course I'm going to do everything I can to maximise my 'return' - I'm funding all of you other users anyway". This is, unfortunately, more of an anti-social networking approach than a social one.
I feel what has been lacking is a shared vision and mission for Steem that everyone can align with. Adding the ability to create more division, without adding a powerful aligning mechanism to unite people is a recipe for disaster imo.