The effect will be minimal but using roller rockers will reduce friction and if they are well made ones they have a more accurate ratio than the stamped steel rockers. This improves the efficiency of the valve train and may realize some power increase. I used some Comp Cam Magnum roller tip rockers with a 1:52 ratio identical to these http://www.compcams.com/information/Products/RockerArms/MagnumRoller.asp

You can find them on ebay all the time.