The do not help preformance. When you first start a cold computer the longer it will take to heat the water the bigger the res.
Once the water has reached a stable temp I would go as far as say they hurt performance alitte, as any water not in the rad isn't being cooled, this goes for long tubing also. It's not much of a differeance and contant flow thru the system will mix it all up.
You can use a T-line to fill your system without a res.