That means that any STEEM that is "invested" in the network via powering up will grow at a percentage of the inflation rate.

No. 15% of the total inflation, which is 8.7% annually at this time, is given to SP holders. 15% of 8.7% is 1.3%, which is how much you directly benefit from powering up as opposed to keeping your STEEM in liquid form in a year. But much more importantly, only powered up STEEM allows one to vote on rewards.