It's a difficult question to answer because it depend on what value you place on defaulted loans. You can:
write them off completely - gives a value too low
use Bondora's method - too high
use the sale value on the SM - probably too low because the market dictates that those buying expect to earn a profit
come up with you own %recovery - will be different for each person
You can use the loan book history for assessing the default recovery rates but you still have to make assumptions about what will happen to long term loans in limbo (still in default but not written off).
I have invested at Bondora for 4 years, but no new loans since early last year.
Bondora says my return is 17.5%, however this figure is reducing every month as the missed repayment amount is increasing rapidly now, although this is still only a third of the total for defaulted loans.
In a year from now I expect to have withdrawn all my invested money, so whatever is repaid after then will be a genuine return.
Unfortunately this does not take into account the tax payments I have had to make!