It is hard to answer you specifically because you seem to have answered your own questions, and so I am confused as to what you are asking.
Here are the facts though.
Larger turbos have more lag time than smaller ones.
Larger turbos have more potential to make lots of horsepower than smaller ones--if the engine can put out enough exhaust pressure/volume to work the larger turbos.
The less backpressure you have with your exhaust, the less lag time you will have, but you can't avoid lag by eliminating your exhaust either.
Keep in mind that the turbine will spin when there is a variation in pressure: in an extreme example, if you had lots of pressure in your exhaust and the same pressure in you manifold, the turbo would not spin. The greater the difference in pressure before and after the turbine, the quicker the turbine will spin. Freeing up exhaust pressure helps get the turbo to spin quicker in that sense.
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