Two friends 'A' and 'B' bought the '03 600's, B was slightly faster (both weigh the same)- but he used 10% more fuel than A. Another friend got the '04- and the '04 was slightly faster than both '03's (I am not sure if 04 is SDI or not). Friend 'A' got the '05, and now he is consistently, and noticeably quicker and faster (top end) than the '03 and the '04. And he gets the best mileage. It looks like BRP is leaning the newer ones out after being cautious..
In fact his mileage has been so good (he always takes 1/2 the fuel of my '03 800 XCR - so around 22-24 MPG CDN! on Muskoka trails), and my wife tried it and loved it, that I spring checked my wife a red '06 600 SDI! (So, can't keep my SoFar PolOnly handle!)... should be getting phone call to pick it up any day now...and with the price of gas what it is - another benefit of SDI....