My dad has the mancini 1.6 rockers on his engine. Solid flat tappet right around .600 lift. Hasn't had any issues, but I wouldn't necessarily consider his engine high HP. Probably making mid to low 500 HP.
know somebody here local with a 264/268 Hughes cam, 620 lift, solid lifter running those Mancini rockers. Car has run mid 10’s and gets street driven.
those rockers are fine for anything but a stout roller cam