The current boost solenoid will never prevent your car from overboosting.. if you free up enough back pressue your turbo is going to run a higher boost pressure..
the solenoid does not know how much boost you are running, it is purely an on/off device, >4500 rpm it opens, letting less air hit the wastegate spring therefore requiring higher boost before it starts opening the wastegate..
using a boost controller is a damned good fix as it allows you to set within ~1psi of your desired boost level.. you just can't do that with the stock solenoid.
Your only protection against overboost really is R&R by the ECU when it detects too much airflow (overboosting)... but if you run say an Aftermarket ECU - you lose this..
Your choice..
I wouldn't recommend changing the wastegate spring, most people would probably recommend putting a bleed valve in or an aftermarket boost controller..
Also - limp mode locks the car to <2,000rpm.. you'll never be able to rev above it if I am correct..
currently the only thing setting your boost level is the restriction created by the hose running to the solenoid/wastegate.. add more restriction or free up the backpressure on a turbo and it's going to run higher boost/or more efficiently..
not sure why it is spiking to 12psi.. what does it go to when you go over 4,500rpm - does it go from 7psi to - 9/10psi?