I'm not sure I agree. What market level returns are you talking about? Beating Propser's average? Bank CD's? The stock market?
It's going to have to beat the pants off of FDIC insured CDs. In order to make up for the dramatically increased risk, you need much better returns.
Prosper loans need to be compared to other income-centric investments. CDs, High-yield bonds, and perhaps even Utilities Stocks. Vanguard High-yield bonds fund has returned 8.7% annually over the past 30 years. Fidelity's Utility fund has returned 9.5% since it's inception in 87'.
As I already said, CDs are FDIC-insured, so Prosper investing should provide a much, much higher return. Prosper investing is at least as risky as Junk Bonds and Utilities Funds. While you don't have capital depreciation to worry about, Prosper default rates are much, much higher than even Junk Bonds, which defaut around 2% annually right now (about half the rate of Prosper AA loans!).
Plus don't forget about PMI's complete lack of liquidity...and the horrible tax implications of Prosper lending returns. Did I mention that those funds I mentioned before have lower investing costs than the 1% fee on Prosper?
We are investing in an unproven market, with questionable leadership, and questionable long-term viability. The entire thing could fall out from underneath us. Many may think it's highly unlikely...but it's possible. We should be receiving returns in line with such risks. We're really, really not. The fact that there has been little of evidence of any institutional-investment interest should speak volumes.
Improved collections is a way of improving returns, as is better verification. That being said, all the examples you bring up are valid. Rates will need to be higher for us to get much better returns.
ETA: And sorry to say...but if those kinds of returns aren't possible...in terms of beating CDs handily and being competitive with junk bonds...maybe this just isn't meant to be. Investment ideas have failed before. This wouldn't be the first.