Alot of variables there. As already stated, a properly sized roller will get the valve open sooner and spend more time using the intake ports mid to high flow rates.
This will only see a gain if the cam compliments the flow of the curent head, compression ratio, and of course a upgraded valve train.
Last season I upgraded to an off the shelf roller cam, replacing a custom mechanical grind. I went .30 better in the 1/4. I am not sure if all that gain was the difference between roller VS Mechanical. I think some of it was the POS "Custom" cam.