I think Google bases a lot of its ranking on bounce rate, i.e. "how often people who searched this were happy with this result." I don't think you need that many hits to establish bounce rate for quite specific searches. Like, is 40 is enough? I can imagine so.

But if you filter for paid members, you've filtered for much smarter people (on average) picking the best results - and you've also filtered out a lot of the SEO people who are going to be trying to manipulate the bounce rate with proxies.