The right thrust is not added in small GA planes just for slip stream or P factor, but to over come the 3 most basic charteristics of an airplane wanting to turn to the left. An of course as you change the angle of climb this will effect all the forces involved
The slip stream does spiral around the aircraft and at slow speeds the slipstream is tight and close together as it slips around the aircraft it will make contact with the virtical stabalizer forcing it to the right and turning the aircraft left. As te aircraft increase speed this spiraling mass of air will elongate out behind the aircraft and have less effect on the virrtical stabalizer