Ok, I think I will make this easier to understand.
Exhaust back pressure is the restriction to flow through the turbine housings.
All vgt's using a stock turbine will have the same ebp readings on the same truck regardless of the compressor. You can only flow so much through the stock turbine wheel and housing. The only one that will lower ebp through the vgt is the elite 59 with larger turbine which is different than their standard 59 drop in.
The same goes for all atmo's using a stock turbine. Except the rcd 75 which has a larger turbine. So, the only drop in atmo that will truely lower physical ebp is the RCD.
So just like anything if you add a 75 to a any vgt it will reduce intermediate backpressure on the vgt. Which will in turn lower total ebp.
Then if you combined the large turbine elite vgt with the large turbine 75 then you would have the most flowing setup. That is all on the exhaust side.
The compressor side is totally different. The air comes in the atmo, then the atmo pushed the air through the compressor of the vgt. At a rate of more that the vgt can flow by itself. That is why they are compounds. If the vgt could produce enough, air then we wouldn't be shoving air from the atmo, and it would be used as a single. In any compound setup, you lose efficiency of the atmo from forcing air through the hp turbo. In a 6.4 with a large atmo and stock vgt then them you are further restricting the percentage of the atmo's capability. Going to a larger free-er flowing vgt compressor allows the atmo to push more air through it easier. Therefore gaining higher volume of airflow in the engine. Upgrading the vgt doesn't actually make more power, it allows the atmo to make more of the power it's capable of making.