has anyone installed a 500 hp efi cam in a 454 mag before ? if so what type performance gain was there speed increase?

The merc HP 450 was a 454 version of the HP 500 Carb. Everything was the same but the engine size. So with that said, you're looking at an 85 hp increase with cam, dart intake, holley 800 carb and gil exhaust.

endeavour: The HP450/500 carb did not have the same cam as HP500 EFI.
Carbed motors had 222/230 duration and .576/.598 lift @ .050 w/110 LSA
EFI had 230/236 and .598/.610 w/114 LSA (very close to Crane 169621)

The 500EFI cam should work well also in a 454, the 114 deg LSA helps better against reversion than the carb cam.

i have a mint low hour HP500 carb roller cam to sell if anyone is looking for one.