My old EcuTek, 20G and the 73mm big MAF worked fine before.. however with the bigger turbo it's exceeding the limits. For reference, my car has an 02 ECU with EcuTek.
From my understanding this is how I believe things work, and why it's now exceeding the limits.
Basically the MAF sensor and ECU combo have a maximum reading of how much air is being sucked into the car. Lets just say for argument sake that's 300 grams. What happens when you run a larger turbo than stock, under boost it can start to pull much more air in faster than expected which "can" exceed the max 300 grams measured. To give you an idea, my car with the new turbo at 16psi of boost was at 280 of the maximum 300 grams that could be metered (NOTE: numbers are just a hypothetical example).
So what was happening with the car, is that it really didn't know what to do with all the extra air coming in, the ECU/Sensor hits its limits, and the car bogs with power dropping off the table.
A trick that is used to try and get around this issue is to use a Big MAF. This increases the diameter of the tubing where the sensor goes, which in turn slows down the speed of the air going past the sensor. You then can make adjustments to your fueling under boost based on your tuning for the extra air while having the ECU still believe it is within the maximum limits.
Some people have gone as far as up to a 100mm Big MAF when using large turbos. The bigger the turbo the more amount of air consumed at a higher velocity past the sensor, thus reaching the limits quicker.
However, there's a downfall to the Big MAF route. Which is around trying to get the car to idle. Tricking the system at boost works great, but now you have VERY slow moving air past the sensor at idle, and trying to get a car to idele with 800cc injectors with what the ECU/Sensor thinks is low amounts of air coming in is really tricky.