I think you all might be interested in the response I got from LendingClub when I e-mailed asking about the YTM calculation:

*Additionally, I am unfamiliar XIRR and it is not what we utilize to calculate YTMs. Please note, we do not provide statistical support and I am unable to calculate the Yield to Maturity manually on your behalf.*

To my knowledge you would not be able to use XIRR to arrive at our figures. Investors in the past that have successfully calculated our YTM have used Euler's method to compute the answer.

In case anyone is curious, Euler's Method is used to approximate differential equations.