I sure wouldn't be drilling that, personally.
I'm beginning to think your problem may lie elsewhere. If you didn't hear a hissing or something when you put your hand over the throttle body when it was running, then there is a major problem somewhere. Unless, you didn't cover it completely. My hand gets sucked to the TB when I get about a centimeter away from it, and it holds on for a second, while the engine stalls completely. Your IAC can allow air in, but I wouldn't think enough to keep the car running with a completely blocked TB.
I would check that IAC, the gasket it has mounting it, the EGR and it's gasket, and the upper plenum mounting gasket.
Remove any mods like the IAC restrictor, whether it be the plate or the copper cap, and start it up.
Make sure the two hoses running to the intake tube are connected, and that the clamps that hold it to the TB and to the MAF are snug.
Make sure the IAT sensor is in place, and the MAF is plugged in.
Look everything over, and make absolutely sure that there are no openings at all between that TB and the MAF.
Start it up, and listen for where you hear the most wind noise. If the idle never drops, while it is running, go to town with carb cleaner. Spray everything shy of the COPs. If the spray doesn't change the idle in any way, then the problem isn't anything you did, it's a coincidence that happened elsewhere on your car. Try a little spray around the lower intake to head connection even. Hell, spray by the coolant crossover. Those hairline cracks can be hard to see on black plastic.
Do you have a tune loaded, or a chip perhaps? Something may have been adjusted, because the last plenum may have had a defect that caused a low idle.
Did you mess with the set screw that holds the new TB blade open at all? The blade should just come to complete close on it's own, and not stick at all. If it's held open at all, too much. If it closes too far and sticks a little when you try to move it, not open enough.
Last thing I can even think of, is possibly the DPFE EGR pressure sensor. There may be something funky going on with that. You should try to get a good video of everything that is happening; maybe we can see something you do not.