Except for those with the K26 turbo. It must come off first otherwise the wastegate will hit the fender and prevent the manifold from clearing the studs.

Preturbo is definitely the way to go. There are way too many variables that can alter temperature drop across the turbine.

I have heard a trick from two pro technicians. Hose clamp about 3 rags over the exhaust tip, start the engine, set it at a high idle (~1000rpm), slowly drill the manifold, and slowly tap the hole while backing out every 1/2 turn to clear the shavings. The idea is that the backpressure will blow out any shavings before they can drop inside.