I missed all of this as the discussion happened while I was on vacation.
In my case, I donate to
RP. I made my first donation fairly shortly after I'd joined, had used some of the techniques, posted to a couple of the challenges, and thought "this place is great". I've made other donations since then.
Had your sponsorship scheme been in place I wouldn't have donated anything. I would have got here, used some of the techniques, posted to a couple of the challenges, thought "this place is great", checked out the sponsorship benefits and decided that they weren't things I was bothered about -- and because you had the sponsorship system wouldn't have felt any obligation to donate money either. Also I have a twitchy reaction to "paid for" sites and virtually never take out a paid membership, while I have donated to several.
Is the amount of money you'd get in in sponsorship payments actually more than the amount you'd lose from people like me (I can't be the only one who thinks that way, surely)? My gut feeling, at the moment, is "no".