You turned up the boost, but by not increasing the fuel supply either through longer injector pulse widths or by using larger injectors (if the stock ones are being ran at over 90% duty cycle. The chip would compensate the rise in boost with a rise in injector pulse width as well as the rise in fuel pressure from the fuel pressure regulator.
I was gonna say there are two things that will make a turbo get that hot. 1: you are running out of fuel causing a lean condition. You have heard the saying lean is mean? Well the extra fuel not only richens the mixture, it also cools the cylinder slightly so you'd have lower exhaust gas temperatures which are fed directly into the turbo. 2: the turbo is oil cooled so if everything else was fine I'd suspect the oil line is plugged up.
Luckily you didn't damage the motor from leaning it out that much, you could have had detonation which if you're lucky would just pop your head gasket(s) and if you're unlucky, maybe bend a rod, burn a hole in a piston or crack a piston.
Anyhow there's a reason why these cars need a chip and not just a cheap manual boost valve. I'm sure you can get away with upping the boost a few PSI with an mbc but at some point the computer won't add enough fuel because it's not in the factory programming to see that much boost. Also I've heard more boost without a chip can cause the computer to cut throttle and you'll lose performance which sounds like what it did to you.
Good luck with the chip, they work well and offer more power and retain factory drivability. Now work on your exhaust or fit some bigger turbos in the car!!