Comments are posts, and are treated the same on the blockchain. So technically, there is no way to differentiate comments from posts (except by with more advanced technical coding, which I honestly don't know if it's possible to perform at the Steem core protocol level).
I do like the idea of comments having a lower reward pool (I believe there was a fork a long time ago splitting comments / rewards pools, then a subsequent fork removing that split -- I'm not sure what the lessons learned were at that point??).
Decreasing the potential value of comments so drastically (or arbitrarily) might be a problem. I'd like to see some analysis from someone like @paulag or the #bisteemit group to look into what the current split between post rewards and comment rewards are. Setting a "cap" on comment rewards, would also therefore "cap" the post rewards.
This could results in an unexpected consequence where Comments are provided with an INCREASE in value as opposed to an intended decreased value in the future (and vice versa with a DECREASE in Post values).
I do like the idea, but I'm not sure that I like the execution portion with the 15/85 percent split. I feel like there would need to be completely different rules for voting on Posts, and completely different rules for voting on comments, and completely different rules for apps that are built on the Steem blockchain that are not part of the "Steemit" culture. For example, Busy.org would be in the same category as Steemit.com, and other similar sites; Steepshot and Dtube would be in different categories - possibly their own? possibly the same?
Just some of my thoughts...