Ok, dubble checked myself and yes, increase spring pressure will increase system pressure. I works the same way as the oil pressure control setup does for the engine oil, once the pressure overcomes the spring tension on the valve, the valve opens and allows flow to bypass back insted of only going to rack so increase spring will allow the system to opperate at a higher pressure. Just remember though, if the pump can not match the volume that the rack requiers then this will do nothing for you. The pump has to exceed flow over what the rack wants or better said, in order to generate pressure, there has to be excessive flow....
The valve and spring is located beind the high pressure outlet of the pump. The high pressure line is threaded into the valve and spring retainer which is threaded into the pump housing. Basically remove the high pressure line and then the adapter fitting that it was threaded in to and inside you will find the valve and spring.