im using 650's and the duty cycle is relatively low, i can also tell you that the car made 258 on the first reasonable pull and to get the car to 320 was a LONG drawn out process. truthfully w/ the new turbo system we've built the car only made 250 out of the box, but what tremendous room for improvement there is now. im confident that getting the car to the levels i want will be pretty easy now that the flow is there. and on the subject of the 550's in your customers car, they would have to be close to being maxxed IMO. and for those who are interested in making there own PE copy get yourself an IHI 34 and mill the housing for a 6.5L diesel wheel and viola instant 1820...blah blah blah, i talk too much...