I have a Garret turbo with a T4 turbine housing and a .50AR trim compressor side. Does it pay to get a T3 turbine housing and wheel or just get a different turbo altogether?

BTW I want to put this on my D-series mini-me stock sleeved motor.

 

I would get a different one all together unless you have a local turbo shop that can rebuild it and change it over. Then it will be like new.