will all this work to make the VNT work on an m-TDI be that much better that the stock 1Z t15 turbo?
The stock 1Z turbo can provide boost fairly quickly but only so much of it efficiently. A VNT-15 can provide more boost even sooner while being less restrictive as boost levels increase. It's not a massive difference if you're comparing those two specific turbos, but once you open the door to a VNT-17 or a GTB1756VK or something then there's some significant improvements.
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I've run a VNT-15 and a K03 on the same engine and the difference is fairly significant. Definitely fast boost with the VNT, but the bigger difference is at higher boost levels and higher rpms. Even at 14-15 psi, the VNT gives noticeably more power than the K03. The usual disclaimers apply...