Smoke only on downhills? 2008.5 Mazdaspeed3

08.5 mazdaspeed 3
Mods: fmic, 3” intake, full 3” tbe, hpfp, tuned. Some other stuff.
Issue is after big downhills while engine braking or even in neutral coasting I’ll blow a good cloud of white smoke, not blue not black. Very clearly white. I want to assume it’s the turbo seals and that I need to swap it, it is stock turbo at just under 150k, I was planning on doing the timing chain as well, but just wanted input on if anyone else has this issue. No smoke on start up or when getting on it. Just on high vac situations
It's an inexpensive job that wouldn't hurt to do. My 89 MX6 GT Turbo had about 135,000 miles on it when I sold it, and its original turbo was still good at the time. Boost was about 4-6 psi higher than factory, so nothing crazy. But you are correct in that there aren't typically too many culprits for white spoke - head gasket, turbo, or PCV valve.