Shimming the pressure relief spring (so that it is tighter) will only increase your oil pressure if the valve was actually bypassing oil at the time. Shimming doesn't increase oil pressure, it raises the point at which it opens and passes some of the oil back to the pan, bypassing the oil galleys all together.
Let's say you are running flat out at 5000 rpm's and your oil pressure is 65 psi. It might be 65 at 5000 because that is the result of the pump's ability to push oil and the mechanical's ability to leak it out, or it might be that the pump's ability to push the oil exceeds the mechanical's ability to leak it and the relief spring is opening up another "leak" back into the pan.
If you have low oil pressure at low rpm's it's not because of the pressure relief valve, which we should really call the OVER pressure relief valve, but because the pumps worn out or the mechanicals are and leak too much.
Tom