Just noticed in the last week or so that whenever I shift under boost my car let's out a big puff of whiteish smoke, most certainly oil since it doesn't smell of coolant. But it only smokes just after the shift, a big puff then nothing, only happens when coming off boost, no smoke when driving, no smoke at idle.

Sounds like the turbo. One of the guys in Kingston here had the same problem with his t25, it would smoke whenever he shifted. He just got a new turbo and no more smoke, the old turbo was fried...blades where hitting the housing.

When the oil seals in my T25 went, it would puff between shifts, like yours, and it would puff if I went in and out of boost, no shift required. I needed oil seals and bearrings by the time I had it fixed. Ended up with a T28 compressor, cause hey, it's off, let's make it better...