I don't want to break your heart but I have seen similar combos as low as 220 RWHP & maybe close to 260 RWHP
the reality is it is just a # , RWHP is the lowest , Net is in the middle & gross as they were rated until 72 was the most optomistic # I can tell you if you can get 400 RWHP on a Mustang dyno you are in supercar territory , I have built a few with 400 RWHP in street cars nothing with M series , AMG etc will be close , you just drive away like they are standing still
I agree on the EFI , I like to see the system run 60-80 % range , you do not want to go lean @ WOT