Per core temps can easily be 5°C ± from center. So you are right on track as being totally normal.
You aren't going to get any better results with a different cooler, it's an i5 and well under the ability of that D15.
TjMax for the 3570k is 105°C, so anywhere after @ 90ish°C you'll experience thermal throttling, at 100°C+ you can expect probable shutdowns. Generally accepted safe load max temps are somewhere around 70°C, but anything under that is all the same to the cpu. Cooler isn't better, or any worse, it's all the same to the cpu.