Using anything other than the immediate hash-rate would cause the clients pools to weigh in on the current speed. Every test shows that immediate hash-rate causes the least false positives and the most accurate results. As for +/- 5% it sounds like a lot of providers rate their rigs at -1% to -5% in order to deliver the promised hash-rate even on slow pools. Keep posting, we'll follow the feedback and see where it leads.
Is this your goal - that everyone rates his rig lower than it can actually deliver? Every provider who rates his rig lower than it actually performs abstains from possible profit. Your current rating system puts even more pressure on the providers as it has an effective influence on leases. From my point of view a strict no-go if you stick to your comment that you want to interfere with rentals as less as possible.
As I posted earlier - trash the current rating system and implement a feedback system for customers on the leaserig website.
May I also add, that the rig is "underperforming" means nothing, since there is variance, and it should be more fair if it was "rig is underperforming right now but was above expected for the last 72 hours".