from what I understand, the oem gt3 plenum is different that the m96 plenums and won't mate to your intake manifolds. the largest plenum you can get that will fit is the 997, and it won't mate to the gt3 tb. so, if you want to use the oem gt3 tb you have to get the ipd plenum.
an option may be to bore out your assembly - pedro offers this service.
otherwise, note that the oem intake design on the m96 is a reducing diameter style (on 986s: airbox to 75mm maf housing to 68mm tb; 997: 83mm maf housing to 76mm tb) this is to accelerate the air via venturi effect as it travels down the intake; the accelerated air facilitates engine breathing at lower rpm and increases torque. at higher rpm the reducing diameter can introduce resistance to airflow, so cars designed to spend extended time at max rpm (ie, track cars, gt3) just use the biggest tube possible. if you look at ipd dynos, cars with this product installed actually have less torque at low rpms but more hp at higher rpms.
so, as posted earlier, I think you might be wasting money and effort making your tb bigger than your upstream maf housing. you will kill low rpm torque without the high rpm hp benefits as you are still doing work to accelerate and then decelerate the air as it passes through the maf housing. you will probably get modified throttle response, however (similar to a sprint booster) where the power comes on earlier in the throttle pedal travel.
I don't think fuel delivery is an issue. I've never read of a 10 hp increase overwhelming the injectors, fuel pump, etc.
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