If I could offer some constructive criticism: Perhaps consider redesigning your turbine exit pipe for better exhaust flow at the turbine face.
Allow me to explain that a bit.
In a turbocharger with a wastegate, you want the exhaust rejoin to happen further down the stream rather than at the turbine face. What happens is that as the wastegate enters bypass, the exhaust impinges on the turbine rear face, which effects the pressure differential between the input and output face of the turbine.
The end result (in common terms) is like exhaust restriction behind the turbine.
Instead of an open adapter plate, make it two separate ports. Then extend the wastegate port into the chamber created to direct the bypass flow away from the rear turbine face and into its dedicated exhaust port. You can rejoin the main flow downstream where it will enter parallel instead of at (essentially) a 90 degree at the turbine face.
You can see and example of this in the BW EFR series turbos. It is built integral to the turbo in that series, but it is the same concept. You can search those turbos and will turn up some basic design concept on the bypass and it's benefits.
You can see here how they've tried to manage the exhaust rejoin with the angle:
Same concept as a separate pipe. Avoid the 90 degree rejoin.
here's the concept on a turbo motorcycle (first pic I found on google) fince we all do better with pictures:
You can see how the wastegate is separated from the main flow. That's for a drag bike (and there's a bit more going on there than we need for this discussion, ignore the external gating) so they just exhaust it to atmosphere (called a "bark pipe" in ricer circles), but a street design would just blend into the pipe for a smooth exhaust rejoin.
You seem to have sufficient skills to build it, you might want to give it a go. Use at least stainless if you do. Gets pretty hot behind the turbine, even after it has extracted the required energy to spin the turbine.
You may elect to not do it though. It is about extracting that last little iota of performance from the design. On an old IDI, it may not make a huge difference and if you're happy with it "as is", it might not be worth your while.
Just something to think about.
I would have thought it was a good bit lighter than that, wow.
