JJ, a ported shroud, like Phil remembers someone mentioning to him as being useful against surge, is essentially a way of allowing boost to bleed away from the compressor wheel of the turbo, what this then results in is a movement of the surge (aka choke) line on those maps ive just linked to over towards the left of the graph, ie decreasing the amount of flow required before surge stops occuring.
The downside is that this will (especially at higher boost levels) reduce the amount of flow from the turbo for any given amount of work done by the wheel (ie the turbo speed).
This then results in a net loss in VE of the engine due to the increased pumping losses of the engine from the increased resistance on the hotside of the turbo required to increase the speed to a point where you get back to the same airflow from the turbo (doubled edge sword this as you also now need more boost, making the intake temps increase which will result in both another decrease in VE, and also a drop in efficiency), and it also will slightly reduce the peak flow potential of the turbo as the safe operating speed of the turbo remains the same, but the amount of flow at that speed is now reduced for the reasons Ive just given.
This all comes back to what I was explaining to you the other day of looking at the turbo (BOTH sides) and the engine as a package, rather than trying to look at items in isolation.