I had this written out so ill post it here:
so the way that the turbo technics VNT is setup is like no other VNT on the market, they use a boost reference to control it. All other VNT use a vacuum controller along with a computer or a dual stage controller. On my new custom turbo ill be using this style as its much much safer and more precise, because it controls the max flow and measures the rise rate and the max boost at the same time. TLDR it uses a constant vac source to control the vanes and the boost reference is used to just align the MAX boost and the rise rate.
that being said, the TT turbo setup is fine for what it is. you can use a boost controller on it the same way you do for the stock wastegate turbo. if you have a composite head gasket (old type) max boost should be about 15-17ish PSI . if you have the MLS later gasket you can do 20 no prob and higher if you are brave.
more boost = more air and lower EGTS for the same fueling settings
You cant convert a TT to the dual stage (safer less spiking) controller without removing the boost actuator and replacing it with a Vac actuator. Frankly its wasted money as for the LR tdi setups it works fine.
here is the controller for VAC actuated VNT (like 99.9% of the market)
https://gfb.com.au/products/boost-c...d-effective-boost-control-for-vnt-vgt-turbos/
here is the controller for the stock wastegate turbo or a Boost actuated TT VNT
https://www.grimmspeed.com/manual-boost-controller/
Both are used to set the MAX boost of either system. you can use the grimspeed or similar to run the TT VNT higher or lower than the factory setting of 17-20PSI depending.