This is a tough one if the wastegate is located on the passengers side header and not on the turbo. One thing we need to know is, what turbo are you running?
There are a couple of does and don'ts when it comes to mounting it onto the header or crossover pipe.
The tubing that feeds the wastegate needs to have the same ID of the inlet of the wastegate. Discharge needs to be the same ID as well.
Make sure the wastegate is not mounted to the header backwards, (The inlet is on the bottom of the wastegate, not the side)
If you have a short piece of tubing that comes off the header and feeds the wastegate, you don't want it to just be at a hard 90* right angle to the main pipe or header. You want the connector pipe to be at a slight angle (IE: 45-60*) to help facilitate a smooth exhaust flow into the wastegate. What can happen is, if the pipe is at a 90* right angle, the exhaust can bypass the wastegate almost entirely and cause you to have a run away or creeping boost condition and or no boost control.
Make sure that you indeed have the 8.7psi Small Blue spring inside your wastegate and not a doubled up Large and Small spring combo. Check this before you order anymore springs. I have seen in the past where the boxes were mislabeled and the wastegate would have the wrong spring vs what the label on the box said. What would really blow would be to order parts that you don't need because you already had the correct spring but it needed to be modified. (IE: if the wastegate had a large and a small spring and all you had to do was take the large spring out.) In a perfect world, to reach the boost levels that you're seeking, technically you should run a Large
Red or Large
Green spring, 11.6psi and 13psi respectively.
It is a must to use both pressure ports on the wastegate to have good boost control. You could have some issues by just running one vacuum/pressure line to the side of the gate and nothing on top. Running your lines that way (only one line) is called Open Loop. Using both ports with a manual boost controller is called closed loop. Here is a picture showing how I have always hooked up a manual boost control valve on every external wastegate setup I've ever owned.
If you have done all of these things, tube feeding the wastegate is same ID and at a slight angle, wastegate oriented correctly, checked to see what spring/springs it indeed has inside, and both ports hooked up to the wastegate correctly, then the last resort would be to replace the wastegate spring.
If you already have the 8.7psi Small
Blue wastegate spring and the lowest boost you are able to see is 19psi and your goal is 15psi, then I would get two springs as knowing which one will be the solution on your combo will take some testing. I would start out with the 3.6psi Small
Yellow spring and see what the lowest boost will be. My guess is it would be around 14-15psi. However, your boost might actually be lower than this. At least in the grand scheme of things it is supposed to be if all of the above items are good to go.
If this is the case and the boost level is now too low and you can't get the high setting where you want it, then I would install the next level spring. This would be the 5.8psi Small
Red spring. With what you have said, as far as where your boost is now when running the 8.7psi spring, one of these two should get you going. The problem you might have with running too small a spring is, the wastegate can open up too soon and cause slow spool up, as well as not being able to reach higher race only boost levels. (20-26psi range) This is why it will take some experimenting with the springs.
Here is the link to Tials website that shows the different springs available for the 38mm.
http://www.tialmedia.com/documents/w3_tial_38_sp.pdf
Hope some of this helps.
Patrick