As Chris says, could be that your wide band needs calibrating or is faulty, however, they have disconnected the std ECUs lambda because the std ECU is trying to negate the fuelling adjustments that have been input to the EMU, because its an N/A the std ECU is set to be in closed loop with lambda feedback for a lot longer/higher load/RPM that the equivalent turbo ECU would, which is I suspect the main problem, unfortunately I am not familiar with using the EMU on an N/A.
It would depend a lot on the turbo you have and when it starts making boost, as obviously as soon as its making positive boost the fuelling and timing will need adjusting, and its this that's causing the problems, but I suspect that whoever is tuning it is not very familiar with boosted N/As and has tried to adjust things too much too soon, but in theory disabling the lambda feedback will help, but first they will need to let the std ECU adjust the fuelling so it will run slightly rich, which is the ECUs default for not detecting a lambda sensor, and then start to make adjustments to compensate, which is probably why its not gone weak.