Measuring boost back pressure

I would build the down pipe the same size as the turbine outlet and no smaller. On my VS 7875s that was 3-1/2. I do think you’ll see an improvement with a 1.25 hot side but only marginal improvements at low boost pressure. Once you get up into the 15-30 psi range it’ll show big improvements as the back pressure will track more in line with boost pressure. I wish you were closer, I have a vs7875 gen2.5 1.25ar sitting here I’d loan you for testing.

On mine I bought the monkey fab vband adapters so I had seamless transitions out of the turbine. V-Band Flange s400 T4 1.25AR (Vs racing 7875, and similar) Turbine; turbo downpipe: 3.5" or 4.0" SS