I think there is a distinct difference between a performing note that has been performing from the beginning and a re-performing note, i.e., an NPN which was worked out to be performing again. IMO, the latter has a far greater risk of default, and the level of risk is highly dependent on the quality of underwriting done for the workout. It's relatively easy to get a borrower in default who wants to keep their home to agree to a workout they really can't afford over the long term.
I have done quite a few workouts which usually include a loan modification, and I am very careful to ensure that the borrower can afford the modification terms I am proposing. I also take steps to make staying on track easier, like requiring ACH payments (sometimes bi-monthly) and establishing tax escrows in some cases. I also make sure the borrower has some skin in the game. Sometimes this involves some payment of arrears, possibly a short payoff. I always have the borrower pay the loan modification fee. Because of this, my rate of default on my re-performing loans is very low, somewhere in the 1-2% range.
If you are thinking about buying a performing loans, my advice would be to find loans which have a good payment history from the beginning with no defaults. If you are buying re-performing loans, you should be looking carefully at the underwriting work associated with the mod, for example, there should be a income and expense statement provided by the borrower. The payment history and seasoning time are also important here. If you are working out NPNs, do a good job of underwriting to ensure that the borrower can afford the payments over the long term.
As far as strategies for getting a borrower to re-perform, it really depends on the situation. The loan is in default and you have the full breadth of options available. I would be very reluctant to mod a loan a second time, I would be more inclined to take re-defaults down the foreclosure/DIL path.