My view - and as always, each to his own - is that if you're building a replica, then you replicate your selected original as closely as possible within you own constraints. The 289 Cobra won a world championship with around 380hp, and 427 Cobras were endowed with 380 to 420hp, so for a 289, 380hp is enough and a 427?....420hp is enough.
But if your choice is 600 or 700hp, then go for it! You would probably do well to buy shares in Goodyear or Firestone though....and they're probably cheap at the moment.
One question though - if you're cruising along, minding your own business in your 427/700hp Cobra replica, and you feel a sneeze coming, do you slow down?
Cheers!
Glen