One of the dyno tests, from 2007 after the atlanta race, courtesy of jayski.com:
#44-Dale Jarrett's car topped the chart with 799 horsepower through the rear wheels. #17-Matt Kenseth's car was second with 798 horsepower, followed by the #99 (792 hp), #42 (788hp), #20 (787hp), #22 (787hp), #2 (786hp) and finally Sunday's winner, #48-Jimmie Johnson, whose car measured 779hp, but that was after his massive burnout on the frontstretch at AMS.
The only horsepower numbers jayski has for restricted engines though, are from 2004;
NASCAR tested six cars on the chassis dyno after the Aaron's 499 at Talladega Superspeedway: #12-Dodge, #42-Dodge, #24-Chevy, #8-Chevy, #21-Ford and #6-Ford. Reports are that the #21 Ford was at 419 measured bhp and the #8-Chevy at 415 bhp at the rear wheels. No word on the rest of the results.