I'm not sure that limiting the filter list to 2 or 3 people would be much different from having free filtering because most people would likely only have two or three people they may wish to filter anyway. Furthermore, it might be worse as it could encourage free users to filter who would normally not filter if filtering was unlimited. You know, the ol' "must be valuable if I have to pay for more, might as well use up the freebies" mindset.
Anything that encourages lots of people to filter would probably impact the system more than a few people with larger filter lists.
My $.02
Once
The best way to convince a fool that he is wrong is to let him have his own way.
~ ~ ~ Josh Billings