have you pulled the air hose of the front of the inlet to check the throttle is closing

iff the idle screw is set with idle too high and there is no load on motor then yeah it will just rev up,

got to be throttle not shut

Engine is a DET. Yes I did pull the intercooler hose off to check the throttle was closing.
Must be a leak somewhere else. I remember it happened while I was coming out of a corner on boost.

Would be a huge leak though as it reaches redline in 2 seconds.

Sounds like intake piping is wrong, or aac valve is stuck open. first make sure that the only hose that bypasses the throttlebody is the one that feeds the pipe that goes to the AAC Idle control block.

Problem was partially severed throttle cable which was catching and causing the butterfly to stay open, but didn't show up when I checked the butterfly!
How annoying.

Glad to hear about the outcome. The less common causes are usually the hardest to figure out.