Helppppp

[Smoke them tyres]

Agree that this could be the cause, BUT:

Be VERY careful with throttle application with either taking the hose off the wastegate or physically locking it shut. Turbos build boost via their rpm and their boost delivery is NOT linear.

Overdoing it with the throttle would lead to a large boost spike and large turbo overspeed.....test just enough to justify or eliminate this as the cause.

Fitting a boost gauge whilst testing would also be a good idea.

Modified turbos can sometimes be lazy due to people roughly hi-flowing them. I would actually call it downright butchery in these cases. A few I have seen involve snapping off each turbine blade tip. Yes, it does improve flow, but it will also cripple the power the turbine can generate to drive the compressor. People who do this should not be let anywhere near these precise items, and should be flogged with the nearest big spanner you can find.