Porting a turbo - much like porting a head, it will slighlty reduce "pumping" loses of the engine and to a lesser extent reduce exhaust manifold pressure (TIP - turbine inlet pressure). It doesn't change the flow maps of the turbo but you will make a little more power. Porting usually refers strictly to the turbine side but can also be done on the compressor. Many times a port job is benefcial at the wastegate to improve its performance.
Clipping - refers to the turbine wheel. The turbine is clipped at the exit (exducer) to allow more flow which helps top end power but will result in slightly more lag. Clipping is a trade off and should be discussed with the turbo builder, items such as A/R (or in our case turbine housing size - 6cm2 etc.) should first be selected and then decide if a clip is appropriate.
There are many good books out there on turbocharging such as Corky Bell's "Maximum Boost".
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