turbos like BIG exhausts. Heck, a naturally aspirated KA24DE made gains across the whole powerband (compared to stock) with a 3" mandrel setup, and at a local dyno, an SR with cams and a stock T25 with a full exhaust made like 30wHP more than an SR with small exhaust, cat, and a T28. Part of that was cams, but at least half was exhaust. Just going from 3" press-bent with a high-flow cat to 3" mandrel with no cat on my little 2-liter turbo engine was noticeable, not so much for peak power gains but from how much quicker the turbo spooled.
But I noticed someone said you were having A/F issues with the ECU... in which case I would most certainly advocate waiting on the exhaust and getting the tuning sorted out. All that extra exhaust flow won't do you any good if you get pinging and hole a piston out