Economically, I think this concept is overall a good one, though it took some time and chatting on steemit.chat to figure out what you're getting at.

I'd like to see it fleshed out a little bit with an explanation of how a user's SteemPower would be affected. For instance, if someone bought into Sp.8 not only would their inflation be 8/104 of Sp.104, their SP (power of voting and witness approval) should be 8/104 of Sp.104 as well. Part of the idea of Steem Power is that people can't jump in and jump out by buying and selling tokens on the open market. It commits people to Steem, and bonds us together. If someone powers up into a shorter-term contract, there should be a proportional penalty to their stake's influence.

The biggest questions are: how feasible is it to change Steem to allow a variable SteemPower divestment time? And how will it affect the performance of the network and the nodes that run it?