Owners of newly upgraded turbochargers often find, to their dismay, that they experience a severe loss of power at the same boost levels they ran with the old turbo. This is because that the upgraded turbocharger has better flow rates and efficiency than the old turbo - that's why it's an upgrade - and the stock ECU/fuel system cannot handle it. Too much air is entering the engine, leading to low O2 sensor readings, high EGTs, even severe knocking under acceleration. The only solution to this problem (besides lowering boost, or removing the turbo upgrade) is to purchase a fuel management computer of some type. These devices allow the owner to 'trick' the ECU and manually adjust the fuel delivery back to correct levels.
Owners which already have a fuel computer installed should run a mimimum of 0.85V from the O2 sensor, and a EGT maximum of 1650 degrees F. Exceeding these values may result in engine damage.