1) Win Goals and Stop Losses do not affect percentage return.
2) They can be of value to those playing negative expectation games because they may help people play fewer hands.
3) The effects of using them are amenable to analysis via simulations.
If you play negative expectation video poker, anything that helps you play fewer hands can aid your bottom line. Pulling money in or out of your pocket, or changing denominations, or having win goals and stop loss rules can all help your bottom line if they slow you down, cut your playing time, and result in you playing fewer hands.
Of course, wearing a wristwatch with an alarm that goes off after a certain assigned time limit, would accomplish the same thing. I'm not sure which, the win goals or the wristwatch, requires more discipline.