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as you have the panel filter type air box, drilling the airbox will allow more hot air into the engine than is desirable, especially since your turbo isn't watercooled.
i would try and source a watercooled T3 from an '87-89 9000 Turbo and associated water lines. this will help decrease your inlet air temperatures before the intercooler, allowing the stock intercooler to do its job better.

next would be the ignition retard stop modification, to retard ignition under boost some more, which also allows you to run higher boost levels.
next, install a knock LED on the APC computer (even if you decide to use an MBC) - pin 19 on the old APC units switches to -ve when it detects knock. so hook up an LED on your dash somewhere, with the cathode soldered to a wire running to pin 19 on the APC module, and the anode soldered to a 680k ohm resistor, which then goes to +12 VDC. this LED will now flash when your engine in knocking.

next, modify the APC unit, or, use an MBC (i personally don't like them) - set boost to a level where ou are just not getting any knock (as shown by LED) - if you arent getting any knock, adjust fuel cut switch under dash, and set boost higher!

before doing all this, make sure your ignition timing is set to 15° BTDC (using timing light), and if you want to run high boost, i found that modifying the ignition retard stop to 7° BTDC (standard is 10° BTDC) at 15psi, allowed for me to hit around 20psi of boost without knock, on 98RON

also, don't forget suspension and brakes. koni adjustables and some good springs, poly bushes etc, all the usual for any 9000 out there wanting to go harder.

also, once you are running over about 16psi, you may want to watch AFR's as the AMM cant cope too much above that sort of air flow. may require a higher pressure FPR, I didnt, but I didnt do long WOT runs at very high boost.
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