1jz boost dropping at high rpm

[shipkiller]

here is my problem. Without boost controller it will spool to 11psi and stay there. With a turbosmart tee manual boost controller with added boost, it will spike to 14~16 psi then drop back to 11 psi by the time I get to 5500rpm (and all the way to 7500).

Then I removed the "gate system" from the boost controller, it helped reduce the spike by a massive margin. Now the system will boost to 16psi then slowly drop down to 11*~12psi as the rpm climbs.

I seem to get max boost @ 3700rpm or so and by 5500rpm it is down to 11psi. The car also started to feel less powerful than before. Compression tested good(175~190 across all six cylinders). I also tested for boostleak and promply fixed them, however I didn't notice any gain in boost or spool time.

The turbos did have a bit of shaft play when I swapped the engine a year ago but they aren't blowing oil.

What is going on, are the twins dying? is it time for a single turbo?

Background info
1989 turbo supra. I swapped a 1jz last year.
here is what has been done to it:
-Driftmotion 1/2jz intercooler kit for mk3
-3" divided Y-pipe
-3" downpipe and exhaust
-6 puck ceramic clutch and upgraded PP
-2jz 440cc
-SafcII
-Mishimoto rad
-New sparkplugs
-Hks Ssqv replica. (does not leak)
-Turbosmart boost TEE installed for parallel setup.

 

[Radial]

The turbosmart Bost-tee is kinda rubbish really. It did not work well on my stock twins, neither my single turbo-setup.
Its in the nature of a Open loop controller; It cant give you a stable boost and nice spool at the same time...but the Turbosmart one is perhaps one of the worst. I cant find one Good review about that product.

I eventally bought a forum review tested HDI SBC-D-SE for ~150$ on ebay, and wollà! rock stable boost.(1 psi fluctuation)

 

[shipkiller]

I used the turbosmart boost-tee on my 7m ct-26 57mm trim with great success.
I do believe there is a problem lying in one of those: turbos assembly/chargepipe/wastegates/actuators. Dropping back to "stock" boost under

 

[Radial]

In that case, you need to check your actuators. pull them off,. bench-test them at different pressures (from an air compressor with pressure regulator perhaps?) and watch the performance.

Remember, two Wastegates demands a more precise boost control, and I dont think the Boost-Tee is good enough for this purpose....and my big 50mm dual port wg was not performing well with the boost-Tee either.

A small CT26 wg might be easier to control, but i'm not trusting that Boost-Tee anymore.