But it's kind of hard to measure it effectiveness
You can certainly measure effectiveness by amount of SBD removed from the supply (which in turn reduces the debt ratio, and therefore the haircut). That is right in the @sbdpotato posts. Currently that's a relatively small number, about 15000 SBD, but it is accelerating over time and should increase much faster now that the SPS funding is adding to the available funds.
When it comes to price, yeah I would agree it is hard to measure since prices (both STEEM and to a lesser extent SBD) move all the time for a variety of reasons. Even if we were to wake up tomorrow and find that SBD had pumped right up to $1, it would be a stretch to say that @sbdpotato was definitely responsible. It could be, but also could not be.