Looks bone stock to me. Although fuel cut is at 11-12 psi on the stock setup. So going all the way to 15-16 is pretty much worthless and the turbo isnt going to last long at those boost level, unless you have an upgraded Ct26. Maybe you have a Lex AFM and 550cc injectors or mabe a Fut Cut defender.
The first thing I would do is check for boost leaks. When you say boost shoots to 15-16 psi and then drops, around to what PSI does it drop to? And also check for codes and go from there. You should also do a boost leak test.
Follow this link, its like a Hayness Repair Manual, but way better
http://www.cygnusx1.net/Supra/Library/TSRM/MK3/manual.aspx?Section=FI&P=23
This link shows you how to check your Check Engine Light Codes.
The first thing I would do is check for boost leaks. When you say boost shoots to 15-16 psi and then drops, around to what PSI does it drop to? And also check for codes and go from there. You should also do a boost leak test.
Follow this link, its like a Hayness Repair Manual, but way better
http://www.cygnusx1.net/Supra/Library/TSRM/MK3/manual.aspx?Section=FI&P=23
This link shows you how to check your Check Engine Light Codes.