I'm sure Jamison's rebounding numbers would go down.
But looking at his last playoff performance compared to Bonner's last playoff performance :
07/08 Jamison : DR % of 26.3%, TR % of 17.7%
08/09 Bonner : DR % of 12.8%, TR % of 9.7%
numbers per 36 in the playoffs
07/08 Jamison : DR - 8.0 per 36, TR of 10.9 per 36
08/09 Bonner : DR - 3.6 per 36, TR of 5.76 per 36
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Granted, that was a great rebounding series for Jamison, and his scoring was down compared to his previous playoff series, maybe due to the work he had to put into the boards, I don't know.
And it was last year, no doubt Jamison will be declining at some point.
But where it counts, in the playoffs, even with the Spurs and next to Tim Duncan, I am still very confindent that Jamison would be a better rebounder than Bonner.