Roller lifters and cams are a huge plus to power and fuel economy. They do limit your RPM to about 6250 because fo the weight of the roller lifter, but otherwise they are great, especially for a street car.
If you go away from the roller lifters, you must also replace the camshaft at the same time. Compared to the roller cam and lifters, you will lose power and economy with flat tappet lifters unless you are running a very radical flat tappet cam. Roller lifters and cams are highly recommended for virtually any application, they generally improve power and economy at the same time.
Ron Lee