You are absolutely right about the spam posting, which is why I say that would be a challenge with implementation. I wouldn't recommend even trying it without thoroughly thinking through how spam posters will be addressed. It would probably require more of an application process for new users, among other things.
I've thought about how narrowing the disparity in value between minnow votes and whale votes might help the distribution situation. On the other hand, the fact that all that whale influence can be bought just as easily as earned is in and of itself a form of leveling the playing field. Theoretically I could become a whale whose upvote is highly coveted and sought after in the next hour by simply buying up several hundred thousand STEEM and then powering it up. Voila, instant whale. The reality for me (and lots of others) is that of course I don't have the kind of money it would take to do that. However, there is nothing stopping me from putting a little bit at a time towards that goal. I right now have a Bitcoin faucet pointing towards my Steem Power conversion address, and I send other BTC to that address as I can. In other words, that's a process I can control and which is guaranteed to work (provided Steemit continues as a viable project for the long haul), whereas getting a large prize on my content is more of a random thing that I really can't control.
My point is the fact that Steem Power (and with it, influence) can be bought actually makes it accessible to everyone. Sure, not everyone can buy a bunch of influence all at once, but everyone who can afford their own computer and Internet connection can surely find a few dollars a week that can be put towards buying Steem Power (or any other "investment" for that matter). Related to that, those 15 cent posts and comments, while not individually impressive, do add up. If I can manage to get three of my comments upvoted to 10 cents each day, then that's an additional few dollars a week going towards my transformation from minnow to whale. This is something every minnow can do. Where in the fiat world do you get this kind of opportunity to actually buy power with pennies?