I think you could strengthen your model for calculating your multiplier quite a bit by adjusting for the fact that people will earn, and give, more over time. As you note in your writeup, assuming people will give an equal amount each year is an aggressive assumption, but I doubt people have good intuition for how much this matters (and its effects interact a great deal with assumptions about discount/attrition rates).
If you’d like, you can borrow the calcs to model this dynamic simplistically from the spreadsheet found in this blog post.
You’ll probably find this cuts your realistic estimate by at least half, though of course that still leaves a substantial multiplier :)
If you move ahead with this project, I’d urge you to try asking different things. It’d be easy to turn this into an experiment. I’d suggest testing out High (e.g. Take a pledge), Medium (e.g. Watch Peter Singer’s TED Talk), and Low (e.g. Check out the EA forum) asks.
More generally, this seems like an interesting idea though I don’t find the logic of the post all that compelling. Tyler, you seem to be conflating two arguments: 1) conversions are extremely valuable and require low conversion rates and 2) building this list and sending cold emails is our best conversion opportunity. I agree with 1 (and have made similar arguments here) but 2 seems to require a pretty big leap of faith.