The SP seems to be the housing with the surge slots. They make it sound good but unless you have comp surge they do nothing, considering they sell the housing without for cheaper, may suggest they don't have this issue. Bigger in and out can only help air flow which is always good a turbo but if you have a 3" intake paipe aleady and don't want to buy a 4"-3" reducer then go with the 3" housing. Unsure how much it will limit power if any.