You guys need to stop focusing on pressure and start concentrating on flow. Shimming the pump and removing the stock filter head will increase flow, but only to the point of the ability of the channels in the block/head have to handle it. Once you reach that point, your pressures will increase rapidly, but you will have little additional flow...pressure is resistance to flow. Think of pressure as flow rate and the size of the channels & piping as flow volume (a simplification)...what you want is more volume. Just removing the stock filter head (and it's associated cooler relief valve), replacing it with a full flow cooler circuit goes a long way in increasing the amount of oil supplied to the bearings, valve train, and oil squirters...you no longer dump part of your oil volume straight to the pan.
Oil pump output (due to gearing) is fixed for a given rpm...output increases as rpm increases. Toyota designed the relief valve to limit pressure at high rpm in order to prevent over pressurization of the system...I've seen at least one guy burst a filter (a cheapo FRAM) due to shimming the pump 11mm. This is a backup function of the cooler relief valve in the oil filter head.
Also keep in mind the primary determiner of pressure is the bearing clearances. Tight = higher pressure, loose = lower pressure. The oil squirters bleed off pressure too...basically they are a controlled internal leak that function at ~40-45 psi. BTW...this is a necessary "leak" if you want to keep your pistons cool
The weight of oil will also affect pressure...a 15W-50 will give you higher pressure, but the high viscosity is the cause due to it's resistance to flow through the bearing clearance...you will actually be supplying less volume of oil to the bearings unless you have modified the motor for it (like IJ did). Using a 0W-30 provides less resistance and you will increase flow through the bearings...you're not pumping a thicker fluid through the same diameter hole.
If you think about it, what determines pressure is inside the block and there are few things you can do to change it. What you want to do is minimize the resistance of the oil flowing to the bearings and squirters to keep the internal parts of the block fully pressurized...full flow cooler circuit (quality components), larger tubing (-8 min, -10 on a big HP motor), and a good filter that's solid construction and flows well. If you really want to go all out, use an accumulator to supply oil at start and as a safety measure if the pump pick-up becomes uncovered due to sloshing. You also want to lower the pick-up in the pan to where it clears the pan bottom 1/4"-1/2".
This is a good read...arz did a test as to when the pump relief spring reaches it's max compression...cliff notes: 11mm will not allow enough valve piston travel to completely uncover the bypass hole...the spring will be at full compression at ~13mm.
http://www.supramania.com/forums/showpost.php?p=601881&postcount=178
This is why I say more pressure is not necessarily a good thing
A 5mm shim is plenty and most of the pressure increase you'll see is at idle, assuming the rest of the motor is basically stock inside. Plus, your oil squirters will function earlier in the power band...that will help keep your pistons cool.