I was thinking more like the inside of the IC. You showed pictures of where the mud was inside the air filter housing, so I wondered if anything might have got through into the IC, and dried in there, reducing the internal size of the pipes.
I would start by taking off the large rubber pipe that runs between the turbo and the MAF, and see if there is any mud in there. If there is, remove the IC, and carefully flush it back through with a hose pipe, making sure to leave it on one end to thoroughly drain and dry before re-fitting.
Also check your IC rubber pipes, in case one of those is split, or not tightened properly, as mine did something similar when the jubilee clip had sat crooked, and not on the pipe at the back, allowing air out. Luckily, mine puts oil through the IC, so any leaks become pretty obvious and messy very quickly.