Yes, the two main things you need to take care of would be the fuel pressure regulator and wastegate.
FYI, under wide open throttle, the 02 sensors are disabeled (open loop). During most other times, they should be enabled and functioning. Some tuners force all-time open loop operation disabling the ECU’s adaptive strategy. While this is not desirable, it typically does work. Your car just won’t have the ability to tune around environmental conditions (elevation changes, temperature changes etc.).
Your car will still adapt with O2's or not. Remember they are narrow band O2's and most of the time with modified exhaust(turbo, longtubes, etc,) their functionality creates more issues with idle and part throttle surges than they help. It will adapt for altitude with the BAP sensor, heat with the MAF and IAT, and coolant temps with the CTS. The 02's are just one small component of a big equation. If all the tables are scaled correctly, the engine will respond correctly regardless. O2's are a nice back up in case there are small variations in a/f that they can address.
On another note, your tuner is getting DC figures from what means?
If he is commanding more DC by adding air to the MAF transfer function, then yes, the DC will increase dramatically, but if the injector opens and only 40 psi or less is passed through it into a pressurized atmosphere(boost), the relative pressure the engine sees is 40 psi - your boost pressure. In your case with roughly 15 psi at 40 psi, you are seeing 25 psi flowing through your injector, and with him trying to band aid that situation by increasing DC, then yeah, it will go to 100% quickly. Does this mean you need larger injectors NO! It means fix all the issues, get the fuel pressure to increase with boost 1:1 and you will be surprised at how pig rich your combo is up top, and as he pulls maf kg of air out, your DC will come down on your injectors. Another issue with turbo cars in foxes is some computers go ape#### when the torque/load jumps through the roof at a low rpm, which can screw the pooch for the tune. I've fought them before.
BBK actually makes a AFPR that is not boost referenced, make sure you don't have that one on your car. As said above, the stockers cannot be beat.