Personally I find the lens evaluations at slrgear.com useful. Using their interactive blur index chart seems to give a good idea of how the lens performs at various f-stops and focal lengths.
According to their review, that lens should perform very well from f/4 to f/11.
That aside I would ask how you are evaluating for sharpness. Reason being is that at f/8 the projected diffraction spot size is around 10 microns. With an 8x12 print there is about 8.5x enlargement making the diffraction spot about 85 microns which is under the average eye sight resolution. But at 100% view on a monitor with a resolution of 93 PPI it results in about a 32x enlargement making the diffraction spot size 320 microns or well above the average eyesight resolution. So an 8x12 print at average viewing distance and eyesight the focus plane should look fairly sharp. But on a monitor at 100% view I would think you will start to see the effects of diffraction.