java.runtime.version=1.6.0_06-b02
StringBuilder[1] 46,349,536
StringBuffer[1] 79,622,829
StringBuilder[2] 40,456,329
StringBuffer[2] 74,662,283
StringBuilder[3] 36,223,249
StringBuffer[3] 72,998,594
StringBuilder[4] 36,400,646
StringBuffer[4] 73,236,472
StringBuilder[5] 36,957,769
StringBuffer[5] 72,561,457

だいたい2倍の違いらしい。