Ok, so what I read is you cannot get more boost than spring rate, right? What is your spring rate btw? And you let the WG open at 130kpa, I assume this is 1.3 bar and not 0.3 bar, correct?
Such issue is typically due to one of two reason from my experience - or both: 1) too high back pressure and not enough force to hold WG valve closed, or 2) leak(s) some where else that vent the air to the birds. Other factors can play in, but these two are the most common
This is how I do it:
1) Regarding testing back pressure and WG function. Absolute best option is to make a metal cover plate you replace with your wg. Yes, you read correct, dismantle the WG completely, put in on the table, install the cover plate, and take a tour in the car. Now you go really easy in 2nd or 3rd gear and build boost, you let the engine get on boost and when it passes your desired target boost level - which is higher than previous boost/spring rate level - you let go of the gas pedal immediately and drive home off boost. If you get the higher boost, you know you have a back pressure issue, and you need more "amo" to fight back on your back pressure, i.e. you need the 4 port valve to help you, and you might need a bigger spring
If you still have the same low boost, then the issue is somewhere else.
If you dont want to make that cover plate, then second best option is to just leave your 3 port boost solenoid home on the table and install the pressure hose directly on top of the WG, and leave bottom port of WG without any hose connected. Then take the same tour, however if running big WG and low spring rate, then back pressure might still open the WG before your desired target boost. It should however change your boost level, but not necessarily close your valve 100% on higher boost levels. A cover plate will tell 100%.
2) System leak test. Simple, make a "cap" to match your inlet hose straight up to your turbo, insert a tire nossle and apply pressure and wait for pressure drop and listen for air leaks. Best is to mount a small pressure gauge. This will tell you where you have leaks. You make a leak test with all hoses mounted to WG as usual.
Example of one of my quick&dirty welded leak test caps, this shows a 3'
I know, it is a bit of work making both a cover plate for a WG, or/and a leak test cap, but it goes into the inventory of high value home made tools for tuning!
And when you have the WG on the table, pressure test the lower port to both read the pressure when the valve crack open, it gives you an idea of the same back pressure needed to open the valve if WG valve is same size as internal membrane. Also you get to hear the air leak you have right here! and see how you cannot hold the valve steady open without keep applying air into the WG. Some worse than others.
You will properly get earlier boost when you fix the issue