ok the 360 was supply contrained for 4-5 months, that lead it lose at least 1 million in sales, in april 06 microsft resolved the issue and 360 out sold ps2, it sold 295,000 units in april http://www.gamespot.com/news/npd-aprils-sweet-16-gives-temporary-reprieve-6151342? the 360 after april it was selling over 200,000 almost every month.
Ok the 360 had supply constraint up until April 2006, and according to VGC the numbers didn't changed that much. On April the system sold an average of 130k units each week, and after that the system start selling a average of 75k-80k each week (with some weeks selling 60k or below) until November when Gears came out. So this supply constraints only help during one month and after that the sales start decreasing.
Also during this constraint period the system sold almost the exact same amounts after the constraints ended (excluding April), they were probably around the 70k mark (with some weeks over 85k), so the situation didn't changed that much, at least not on a big scale. All number are taken from VGC.
Nintendo and PC gamer