Looking at the four CFAR questions (described here), accuracy rates were:

74% OB folks ("Been here since it was started in the Overcoming Bias days", n=253)
64% MoR folks ("Referred by Harry Potter and the Methods of Rationality", n=253)
66% everyone else

So the original OB folks did better, but Methods influx is as good as the other sources of new readers. Breaking it down by question:

Question 1: disjunctive reasoning
OB: 52%
MoR: 42%
Other: 44%

Question 2: temporal discounting
OB: 94%
MoR: 89%
Other: 91%

Question 3: law of large numbers
OB: 92%
MoR: 85%
Other: 81%

Question 4: decoy effect
OB: 57%
MoR: 41%
Other: 49%

One possibility would be for Eliezer to ask people about it in his author's notes when he updates HPMOR.

On the second reading, I realize that I'm asking about HPMOR and spreading rationality rather than HPMOR and community building.