There are no SIP coefficients in PinPoint, instead the system we have pre-dates SIP and does essentially the same thing. I can't guarantee anything about the accuracy, you will need to test with your data and decide whether PinPoint is "good enough" or not. The average residual does take into account the distortion. Everything takes into account the distortion And note that "distortion" is the departure from the planar projection you select (usually TAN) not the total "distortion". In other words PinPoint produces the best fit WCS then applies additional corrections to the best of its ability. The XY origin is the standard FITS conventions, of the upper left. You will learn these things if you decide to use the API.